Andrew R Stewart 

I'll toss out some comments that show my biases.

Seat angle has, basically, no effect on bike handling AS LONG AS the seat set back is the same. It has a little bit of tire clearance effect but as I'll mention later i hope you wouldn't be shortening up the chain stays by enough to run out of tire/seat tube gap. The seat tube angle does very much effect where on the seat rails the post is clamped for the post that you're planning. So, and you mentioned this not I, if you wanted more cush and you then thought that a sprung post was the way to go then a straight post/seat tube angle would be best. because there are more sprung straight post options the set back ones. Either way getting whatever post to clamp the seat rails in their center is my goal WRT seat tube angle (and set back being maintained).

Chain stay length has a few effects. It will help control the front/rear weight balance and this is the largest handling contribution CS length has IMO. CS length also effects the tire clearance as said above and it contributes to a good or bad chain run. The shorter the CS the more chain rub, noise and general rough feel the chain will have.

BB height is about cornering clearance (crank arm length) and handling feel. The higher the BB the more it will feel like you're "falling" into the turns. However the stability is another thing and has many disagreeing about BB height and it's effect here. I would say that other geometry aspects have FAR greater effect of stability and cornering then BB height.

The Op not having a real good understanding of all this is all the more reason to follow the builder's lead in most all the design aspects. He says he "knows the feel" but not what has contributed to it. The advice to list/refer to a few bikes that met his goals is spot on. While putting in writing what he wishes, handling wise, is not wrong it is going to be full of descriptive judgments with little actual fact to grasp and discuss.I always thought that some of the bike reviews in consumer magazines are questionable. When two bikes with the same core set of geometry dimensions are described so differently I tend to question the subject description process. Hence having actual bikes to reference is very important, including bikes that the OP has found to not be what he wants.

In the end some of the OP's job will be whether he wants a subcontractor or a custom builder. While you do need to mention what's important, the builder also has their standards and wishes of customer control that might not match yours. (As an example- I would never build a bike with a sub 41cm chain stay). Better to discover this before the builder feels the need to charge consulting time.