The reticle on the TAO-1 is set up for M855 from a 20" barrel mounted on the carry handle.
Actually, I believe the TA01 is set up for M193 (55gr) from a 20" barrel mounted to the carry handle and TA01NSN (and all other flattop mounted ACOGs) are set up for a 14.5" 62gr M855 from a TA51 flattop mount.
I'm wondering if those will hold tight and not come loose?
As noted, you'll need to use loctite or vibration from shooting will cause them to work loose. Also, the second screw mount is only available if you drill a second hole in your carry handle; but I have found that as long as you use a dab of blue loctite, the single screw will do just fine.
Also as El Tejon noted, you'll increase height over bore issues in close and for some people the height of a carry handle mount is an uncomfortable cheekweld. I used mine in an A2 for some time but got tired of having to rezero every time I removed the ACOG and the rifle didn't fit in several racks/cases well with the ACOG mounted on the carry handle.
I eventually went to a flattop receiver and an ARMS #19S mount.
So the way I'm thinking is if I get it dialed in at 100yrd it should be accurate at 200, 300 etc. Is that right?
A couple of things to remember - first is that the ACOG is set up for specific barrel lengths, ammunition and mounting locations. If you change just one of these, it won't affect the reticle that much for 99% of your shooting. If you change several of these, you may need to tweak your sight-in procedure in order to better match the reticle. There is a very long post in the optics forum at AR15.com that deals with this; but that site is down at the moment and it is too much information to duplicate here.
Second thing - the ACOG is set up in meters, not yards. You can still zero the ACOG at 100yds; but at longer ranges you need to remember that since 109yds = 100m, you will see some misses if you shoot at a 600yd target (550m) using the 600m hashmark.
Finally, at less than 300yds, there won't be a lot of difference. At 100, there will be even less.