The sun dries the surface of my seedling and I stupidly think the whole tray needs watering again.
Everyone has this problem when they change mix - it's just a question of building up experience.
The quickest learning curve is to get a Ph/Moisture/Light Meter - the square one I have can be bought for £2,3,4 or 5 (the latter from Wilko, with P&P at £4 but free if collected), there is a round one too - they cover 2 pages on Google Shop.
Cheap and cheerful, but as long as it's working it will prevent mistakes.
Cheers.