Homemade soil mix

So looking for some advice again on a home made mix for my giant pots. I’ve done a lot of reading over the last few days about the difference between compost ,topsoil and soil based growing mediums. Some of you may have seen my post on my plan to use some of my existing medium clay mixed with my go to compost (verve peat based) to make a good medium for my giant pots for gunnera and some other things. After a second look at what I have I’ve decided against using some of my existing soil due to a plan change meaning I won’t have this soil spare. So on to my actual question, having never used the John Innes 3 recipe soil would this be good to mix into the verve compost to make my own blend? Or is the number 3 and the verve stuff to similar.I’m thinking something like 125 ltr of verve to 75 ltr of number 3, or would I be better off with bagged topsoil mixed with the verve compost. I’ve had such good results with the verve stuff in my gardening career but need this mix to hold onto the moisture a bit more than I would normally like for my tropical plants.

 

I usually mix for general use, 60 litres, peat based compost, "jacks magic" probably similar to "verve", 30 ltrs JI no3, and 25kg bag of builders sharp sand.
Chuck it all in a concrete mixer.

But for certain plants I add osmocote, when potting,more sand, or grit, depending on the plant.

I'd steer clear of using top soil in potting composts as it wont be consistent.

 

Thanks for the reply never thought of using my cement mixer for this, was going to mix in barrow but this will make life so much easier. Have a ton of sharp sand aswell from doing my mums driveway so that’s a bonus.