Yes there two different things, go with whatever the spring manufacture you buy recommends. That would usually be a air gap setting.
My 2014 Veek took a litre for both fork legs ie 500milltre per leg for a 135mm air gap.
Try Hagon or Wilbers. Yes they are progressive but not the same as say hyperpro in that only the top 40mm of the spring is the progressive section where as hyperpro it's more like 80mm.
They feel good on the road and don't top out like some straight rate springs. Hagon are the cheapest and I've used them many times with good results, in fact better results than the few times I used Ohlins.