So does anyone have a good formula for taking target lure size and deriving what the blank lure range should be to cast optimally?
Like the 27 times the diameter of the reelface for locating first running guide.
Or is that lure rating given by manufacturers such an arbitrary marketing driven number that any formula become meaningless?

Generally, the optimum casting lure weight would be smack in the middle of the range given.

This was covered pretty well in Volume 7 #1 in the surf rod article. Any blank will cast a range of weight provided the caster adjusts his power input. In other words, if you see a blank rated for say, 1/4 to 3/4 ounce, the user should recognize that to get the same distance at opposite ends of the range, a little more power will be required to cast the 1/4 ounce lure, and a lot less power will be required to cast the 3/4 ounce lure. Full power on the 3/4 ounce lure will likely overload the rod, while very little power on the 1/4 ounce lure will likely unload it.

The sweet spot is usually, although not always, found smack in the middle and requires only moderate effort on the part of the caster. Of course, these ratings vary from maker to maker.

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I asked because i had always assumed the middle was the sweet spot, then test cast a rod last night that was rated 1/8 to 5/8. I built it as a light topwater but the owner was telling me how well it threw a large zara . I cast it and it loaded effortlessly and not like an overloaded rod at all. Popular good brand blank too.

The more weight, the easier the rod is going to load and the less effort required to cast it. As long as it's not being overloaded, that is.

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