Weight
Weight is a factor also. A much heavier plane will glide further.
I think it's very important - as was mentioned - to go through the mental gymnastics of working out reasonable "how goes it" gates for every descent. You can work out the descent rate to a gnat's ass - and be totally surprised when you get to the bottom of a descent path, if you didn't check a couple of times along the way. You can make very fuel efficient adjustments on the way down - and avoid that - "Uh - I don't think we can make this next altitude", or drivin' it in at 3000' for 50 miles.
If you do the math consistently, it becomes second nature.
That being said it is also important to find out the likely descent rates for your A/C as you are doing.
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