When I mix soils I do it based on volume and not weight.
So, tentatively speaking, I would use this as a base:
2 parts soil
1 part peat
1 part perlilte
But.....
What brand of soil is it? How does it feel? soggy or dry? I can get organic soil from the cypress bogs that is almost dripping or some dry pine bark crap from Miracle Grow and the ratio for each soil would be completely different. Do have anything to buffer the pH? Are you adding any composted manure or other amendments?
I think you have an acceptable base to start with but it could be much better with very little effort.