Correct Wholesale Formula

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Hello I'm new to the real estate business and my goal is to become a wholesaler. 

I'm wanting to understand the correct formula in order to offer the right price to seller and buyer. But I'm having some confusion as to which formula is best to use:

Everywhere, I keep seeing this basic formula: 

ARV x .70- repair cost= price offer to buyer

THEN subtract your fee from buyer price which = price offered to seller.

BUT at a REIA meeting, I had a great mentor tell me to never work with %'s only work with numbers for my formula such as:

ARV- Commission- utilities- taxes-HOA-repair-costs and a couple more I'm missing THEN subtract my fee for a true offer to buyer and seller.

Please help,  which do I use?