Sure, here is how I came up with those numbers:
For a house with a $270,000 ARV the minimum spread he will accept is $45,000. Subtract $45k from $270k and you get $225,000. This is the maximum the investor can have in the property to achieve the spread he wants. $225k has to include the purchase price, repair costs, holding / selling costs, etc.
He said the most he will pay is 70% of ARV. If ARV is $270,000 then his max purchase price is $189,000.
$225k minus $189k = $36k. $36k is what he can spend after purchasing the house to achieve a spread of $45k IF the house sells for $270k. Keep in mind the $36k needs to cover repairs plus any anticipated holding and selling costs.
You can plug numbers into these formulas to get to the same result.
ARV – desiredSpread = entireCost
ARV * maxPurchasePercent = maxPurchasePrice
entireCost – maxPurchasePrice = allowedCostsOverPurchasePrice
$270k – $45k = $225k
$270k * 70% = $189k
$225k – $189k = $36k