moeee -- i wrote the spreadsheet just to answer your initial query -- the formula is very similiar to your approach but the amount/bet is based on a predetermined outlay (like dutching)

would i use it? -- NO
why not? -- i would not be confident that my rating-price could produce overlays in a ~120% market nor would i be confident in the magnitude of any overlay -- the penalty for getting it wrong (big overlay) is severe and would quickly lead to the poor house
what would i suggest? -- if i was going down this track i would use my ratings-prices to "identify" any potential overlay bets then just dutch them

good luck

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