Negotiating Sales Price in the Small Deal Context

I have been looking at a bunch of small deals lately (i.e., SDE < $500K) where the owners and sometimes family members do substantive work in the business. On the CIM, the owner(s) will add back their pay and that of family members and report a high SDE level. When it comes time to create a financial model for the deal, I have added back the owners’ salaries and then added in necessary expenses to responsibly cover the replacement cost of the owners and family members. This creates a challenge, however, for arriving at a valuation for the business because the seller wants to add back all the owners/family pay, but a new owner will need to replace that labor and SDE will necessarily be less than the seller believes. For example, say the SDE is claimed to be $500K, but an owner and her husband work in the business full time in executive roles. For argument, assume the value of their work is $100K each. The seller will argue that SDE is $500K, but if I find replacements for them, SDE will be $300K for me. With a 3x multiple, that’s a sales price difference of $1.5M vs. $900K! This creates a wedge between the seller and me because while we may agree on the SDE multiple, our valuation will be sizably different because we calculate SDE differently. I think part of this comes down to differences in operating choice: being an owner operator vs. just an owner. And also that the seller is thinking in terms of SDE and I’m thinking in terms of EBITDA. Would anyone be able to educate me on (1) how I should be thinking about this problem? AND (2) how do I approach a seller/broker to get them on my side and agree to a lower sales price? For example, do I try to argue SDE or do I accept their SDE and argue for a lower multiple? And apologies if this is a stupid question. I am trying to educate myself! Thank you!