50% compost may be too high, although I don't know anything about mushroom compost. Worm castings are great, and some peat moss or coco coir would be good too. A more standard mix may be more like equal parts perlite, worm castings, and coco coir or peat. Or like 50% perlite, then equal parts worm castings and peat or coir. I have a hunch that it wouldn't be a good idea to use more than 20% mushroom compost, but then again I don't really know. Have fun mixing up your soil