What is the best size latex to use on a tube call?
me it is all in preference I prefer the .004" as I can get rasp and keekees at the same time. the downside is it is thin so delicate measures need to be taken to insure it remains pliable. I suggest a plastic baggie to store you call in with the reed in place out of the sun light. If you chose heavier reed material say, .006" it is stronger but the reed will need to be placed on the call in a looser manner for flexing.
For latex I use Thera Band exercise bands on my calls, you can find it in drugstores or online. Light color is .004, yellow is .0068, red is .008. It's cheap so see what suits you.
Personally I prefer yellow
http://www.protherapysupplies.com/Shop-by-Brand/Thera-Band-Exercise-Bands?gclid=CMTPuITV_rICFcmd4AodvWsAbg