Advice on potting mix for trees - how much fungal compost is too much?

I have an abundance of (free) fungal compost from decayed arborist woodchips.  I also have red clay topsoil.  Any other ingredients (i.e. peat, vermiculite, sand) I have in limited amounts or have to purchase.  Any advice on a tree potting mix that utilizes my free ingredients most effectively?  I am potting up some young figs, mulberry, and aronia berry. Thank you in advance!