Plenty on this if you look in the archives, but FWIW I tend to ignore the dipstick that's in the cap on top of the swirl tower (the plastic indicator on the end of the dipstick is the same colour as the oil and makes it almost impossible to see especially if you've got a tank of fresh oil)
Following advice on here, I made up another dipstick from thin alloy bar (B&Q) long enough to go all the way to the bottom of the dry sump tank with marks on it at 9.5, 10 and 10.5 inches from the bottom. Run the engine, get the oil warm, switch off and immediately measure the oil depth - each engine will differ, but mine runs happily with a 10 to 10.5" depth of oil and none being thrown to the catch tank. Make sure you get the stick all the way to the bottom, there are one or two projections in the tank.
No point doing this on a cold engine or one that has been sat for a while as some oil with naturally drain back to the sump pan and give a false reading.
As for refilling, just pour oil into the tank via the swirl tower. If you do drain the oil, never refill the engine via the yellow screw cap in the cam cover - when you start there will be a delay before oil pressure is established (due to the fact that all the oil will be in the sump and it takes time for the scavenge pump to suck it all out) and no oil will be supplied to the engine = ouch ☹️
Hope this helps
SteveP
The Caterham Pages here