What caliber are you trying to reload? Do you already have bullets and/or powder? If you post some specifics the people here can either tell you from direct experience what the charges and dimensions need to look like or point you to a reliable source of load data.
A good idea, I would just mention to always check data from the internet against published data. People here are very good and the mods do an excellent job but it is easy to make a typo.
Some powder companies list start and max charges for their powders Western (Ramshot/Accurate Arms, Hodgdon) are examples, some just list Max charges Alliant is an example.
If all that is listed is a Max charge general rule is to reduce by 10% to get a start charge.
I always like to check the powder companies web site, they did make (or do market) the powder.
Since your starting out be aware that OAL will affect pressure. The deeper a bullet is in the case the less space there is in the case, less space in the case = higher pressure with the same charge.
To give quick example going form a 9mm round with the QAL a 1.10 to 1.06 inccreased velocity about 50 fps. About the same as if I had increased the powder charge .1 or .2 gr on the 1.10 load.
Different thoughts on varying start/max charges in manuals.
Lots of things can be the reason they vary, a few are different lots of powder may be a little different, different test temps, different barrels, lots of bullets, bullet type/hardness/bearing surface, primers etc...
1. Line of thought is say I have start charge 10 from one source an 11 from another source with a max of say 13 and 15. OK average 10 and 11 and get 10.5, average 13 and 15 and get 14.
2 A different line of thought is use the lowest numbers, err on the side of caution. So say 10 is your start and 13 is your max.
What is really the MAX for you gun will be different than what is published because all guns/ barrels are different.
Start low and work up. Erring on the side of caution is not a bad plan here.
Not directly addressing your question but it is always a good idea to only have one type of powder out at a time.
Mixing up data and powders is a recipe for an accident.
Simple example say some .223 load calls for 23.5 gr of H335. 23.5 gr of some pistol powder say HP38 (which sort of looks like H335) would more than likely blow up the gun with possible shooter injury or death.
(I say more than likely because I have not tried it to see, but I doubt the rifle would survive it.)
Be careful, come up with a plan to prevent this from happening. It does happen.
Usually the most accurate load is less than a MAX charge as other posters have noted.
Be safe, have fun.