Capex seems like its calculated wrong

[Kris L.]

So I was reading the BP book on rental investments and it tells me to keep a % of rent aside for this, but that makes less sense to me than a $ amount.  For instance, if I have a 1200 sqft 3/2 in a class C and a similar size in class A that rent for 900 and 1800 respectively, actual capex $ isn’t going to be that different.  A roof would still be about 4-5k, a water heater will still be around 600 etc...  and lifetime shouldn’t vary much between them, so why keep half the $ for the class C?  I mean maybe the appliances would be a bit more in the class A, but Why not just find Replacement cost and lifetime for the water heater, roof, appliances, water softener, HVAC and figure monthly avg for those instead?