The left thumb of the support hand can hit the rear switch easily, although there is NO constant on without twisting the tailcap. If set to just where the light is barely off, your thumb can apply the slight rotation needed for constant on. Mine is high to clear a side mounted sling swivel. Where you DRILL the two small holes is entirely up to you. Closer to 10 oclock is doable with a normal sling.
Handle your weapon and observe where your thumb is naturally located, and try to have the tailcap switch close to where it needs to be. Again, the paddle switch and shorter overall length of the TLR-1 will be more user friendly. IF you do run a side sling, some users just bolt the short rail to the forward handguard vent holes at 6 oclock. OK on a rifle, IMO not enough room on a carbine, although I have seen some TLR-1's run that way. My "heavy Zombie rifle" uses a TLR-1 at 6 oclock for backing up a P91 based light (soon to be a M3T based unit!).
If you want an inexpensive lighting solution AND the dual light redundnacy I favor, the Thunder Ranch setup is decent. If you want better ergonomics, particularly with a carbine, an offset light mount and vertical grip feels much more natural than the TR kit. A railed handguard opens up many more options.
Although I prefer using higher end offerings, I DO own a pair of TR mount brackets and would not have any issues with their duty use as long as appropriate training/practice with the system is invested PRIOR to a serious encounter. For indoor use the TL-3 is overkill, and I felt even the humble TL-2 LED was adequate for mounted use.
Being able to remove the mount, leaving just the unobtrusive short rail installed is a strong feature, especially for owners who don't leave their rifle geared up during casual range sessions.
The DVD is entry level material but still very interesting, and Clint makes installing and removing the light look super easy. If you already own the kit, it will serve you well. If you are still shopping, strongly consider getting just the short Streamlight M16 mount rail and a TLR-1 for a more refined solution.
Paladin
ETA: maybe also look into the new Streamlight Strion tactical light, which mounts like the TR kit, but is rechargeable.