Optimum enclosed porch humidity level?

[Biiilllll]

I enclosed my screen porch this winter... added lots of CFL's to keep my many various orchid babies happy.... most are doing well even though it is cooooold outside.... a couple of heaters keep the nighttime minimum low temp to the mid 50's... phals are inside where it is warmer.
I have a couple of humidifiers going and usually have humidity levels of 50% to 70%.
Question.... what would you suggest as the optimal humidity level? Thanks for your help! (I have sooooo much to learn!)
Also.... quick plug... don't miss NCPOS show 1/14-1/17 at Daniel Stowe Botanical Garden in Charlotte NC... spectacular facility and lots of very experienced growers exhibiting.