I just sell for w.e it sells for and keep dropping prices even if I sell for a lose, ive found the key is to keep everything moving through the sales process. I keep 20-40 slots filled just about ever chance I get. Since that's your sales cap why waste it deciding on profit or lose. This cuts the time selling things and gives me some more time creating and gathering materials.

It works for me I start at release with 100k gil and since then ive saved up 2.5mil gil while also leveling BSM , WVR , CUL , ARM all from around 10-15 to 50 and currently turning a profit on everything Im making for LTW and ALCH as I level those up to 50. So I feel no pressure or worry at all about undercutting