All clay based soils do not have a drainage problem. If I am in doubt about drainage, I do a simple percolation test. Dig a hole somewhere close by about 18 inches deep. Fill it with water and let it drain completely, then refill again and this time keep track of how long it takes to drain completely. If it does it in 5 hours or less, your drainage is OK. I do not mix compost with my clay, I do mulch regularly out to the drip line. Al

Thanks! Is that considering evaporation too or is that negligible anyways?

In this case, evaporation is negligible.

Ok I'll try that :) but I won't be able to do it by the tree with compost clay mix. I'll do it in the solid clay.