Let a be the top left number of a bow tie shape. Therefore a is one of {5,6,3,2}
If you want ???=100 then calculate ( 3/2*a^2 - 107/6*a + 239/3 ) * a
If you want ???=118 then calculate (19*a^2 - 89*a)/6 + 135
If you want ???=690720, the moon landing date YY MM DD, then calculate ( ((9668466 - 690602*a)*a - 43508104)*a + 62155800 ) / 12
If you want ???=x then calculate ( (((14*x-1614) - (x-118)*a)*a - (63*x - 7256))*a + 90*(x-100) ) / 12
You can use that last one to make the answer anything you want. You might want to simplify some of the numbers if you use this, for example with the moon landing:- x=690720 and therefore, (14*x-1614) = 9668466, (x-118)=690602 etc
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Something similar could be done for any of the corner numbers individually, or for many different combinations of the corner numbers. BUT probably the author had a particular method in mind that is very simple (having few steps, and/or having numbers that can be easily done in the head). Trying to guess what this method could be isn't very interesting unless it can be seen fairly quickly (to me at least). All too often the author throws something silly into the puzzle like, "if the top left number touches one of the squares in the background then add 3 to it". For this reason you're unlikely to get the helpers here spending a lot of time on this. So now I'll leave others to ponder it (if they are willing)!