Honestly I can't say what mags I used. I got them from a place that sells used gun parts that 2152hq suggested earlier in this thread.

https://www.libertytreecollectors.com/productcart/pc/viewPrd.asp?idproduct=1678&idcategory=62
.

I just asked for magazines that fit an SMLE #1. I just used what they sent to me.

And I have no formula for the spring. I just cut it a bit at a time until it worked. It's not very difficult, but getting the follower and spring back into the box is something you need to learn. I find that if I tip the follower a bit to the side I can insert them easier.