Hard to say but maybe about 1/3, then the next 1/3 (the middle bit) would be the mid-zone with the last 1/3 being the warm side. Yes you do need clips to keep that lid secure. Anything more than about 1/8" give will allow the snake to sneek out. We just had a huge scare last week when our 3,000 gram plus female snake, Brannagh, got out. I would have sworn her lid could not possibly have enough give to allow this and she's been in that tub for the better part of 2 years but the big goof managed it. Thank gawd for a secure snake room but you can bet your last buck she's got more clips on that lid now than before. If you don't own one, you might want to pick up a cheap (around $6.00 or less) soldering wand to melt in your vent holes.

Once you have all your supplies, set the tub up with heat source running, substrate of choice, hides, full water dish...basically the whole deal without the snake. Let it sit for 48 hours and watch how the tub settles. Then you can do whatever tweaking you need prior to putting the snake in there.