hiya and welcome to the forum!!!!
I use powder tablets so it's easier for me to divide them up?
however, on the powder box it should tell you how much to use (according to their recommendations
) just use a 1/4 of wht is stated on the box
40 is fine (in fact most pockets state 40 on the labels!) if he gets a tummy bug or the poops then wash at 60 to make certain sure you are killing germs.
You can do a random 60 wash every now and them if you like but it's not necessary!
Don't use fabric softener! Try not to tumble dry the pockets if you can avoid it as the heat in the dryer can wear out the pul (although apparently a tumble every now and then helps them to retain their waterproofness but I have never tumbled pockets and never needed to!)