Power factors in IDPA and IPSC/USPSA competition are not ceilings, they are floors. Your ammo must be a 'combat' or 'self defense' loading. Generally, this is a small velocity/energy range that is capably of producing the maximum projectile action (expansion) in a human sized and structured target. Insufficient velocity/energy and one is penalized or disqualified from competition. Any excess power is a waste and a detriment felt in extra recoil and muzzle flip.
Gun games are 1911 centric and the 1911 platform is used for a wide variety of gun games. Target games like Bullseye use very light loadings of medium to heavy bullets for paper punching accuracy, Pin games-like shooting bowing pins and other heavy objects to knock them from a table require very heavy bullets and near maximum charges. Hunting uses range from very light to very heavy in both powder and bullet weight-run in opposing direction, mostly.
To make a minimum power factor (floor) for COMBAT GUN GAMES requires that an average of some small number of bullets, shot under match conditions generate a number GREATER than the power factor (floor). One or two alibi rounds may be allowed, replacing shots generating a lower velocity and the computation redone, but results are definitive and irrevocable under the game rules.
Long story short: you need to DEPENDABLY generate a statistically derived small sample average power factor number greater than the power factor floor for all range and atmospheric conditions where you will compete. Shoot for an average power factor number of power factor floor + 10 or 175000. Until you gain some consistent statistical numbers on your ammo, load/shoot for 175000+ (i.e 175200 to 175800, typically).
You need a well recognized (as safe, sane and reliable) source of information. Universally, there is good agreement that the Speer Reloading Manual (12,13 or 14) is the most up to date and accurate.
All the lead bullet loads (except certain hand cast hard 44mag loads)for typical defensive caliber handguns (9mm, 38/357, 40/10mm, 44, & 45) are TARGET LOADS-soft lead. Conversely all jacketed loads are combat or defensive except where noted as 'HUNTING' (which are intended for creatures heavier in weight and construction than humans).
Your data lies between these extremes. It's not listed but it is implied in the forward text strategically placed about the manual.
To achieve a 'combat' loading in 357Magnum that just exceeds the major(IPSC/USPSA) or ESR(IDPA) power factor floor you must use a hard cast bullet (Missouri Bullet, Meister, etc) and extrapolate data between target loads listed and a safe weight 0.5 to 10. grains below charge weights listed for a similar weight jacketed bullet; both using the same powder. Hard cast lead does NOT make for a good defensive ammo-its strictly a gun game lower priced alternative. The Missouri Bullet website lists and demonstrates the required bullet hardness for a particular bullet velocity target; this hardness number can be compared against other bullet makers if they reveal their hardness standards.
To do this deed efficiently and safely you MUST have a quality chronograph and you MUST be able to recognize the first signs of excessive pressure as exhibited in the spent casings.
Pay close attention to using the Speer manuals handgun data section (as opposed to rifle) when developing these loads. Pushing 158 grain (soft) lead with Unique at target velocities requires between 5.5gr(970fps/6inch S&W19) and 6.0gr(1035); 175000 power factor target is 1108fps. A 158gr jacketed bullet runs 978(6.9gr) to 1040fps(7.7gr UNIQUE). While neither pair of loadings meet your requirements the jacketed maximum is well above 1200fps), your probable combat loading for a hard cast bullet is 6.1 to 6.5 grains but under no circumstances should you exceed 7.2 grains of powder. YOU MUST CHECK AND INTERPRET YOUR PARTICULAR CASES FOR PRESSURE SIGNS AND ALL THE USUAL DISCLAIMERS APPLY-User assumes all risks.
Special Note:With other powder-particular newer 'Magnum' powders, there may be little or no significant gap to extrapolate across and in fact, these powders may make 'major' with a target loading IN YOUR GUN.
Personal Note: I've competed in both events, but stopped some time ago. Rules change and there is something about 9mm being restricted to minor power factor in USPSA/IPSC-38/357 can be 'called' 9mm so a check with current competitors and IRO types in USPSA should be made prior to pushing 357 to major power factor-it may not be legal for competition.