I need some "pro"fessional input on average handle lengths for various spinning rods. I need to break this into about 3 categories i.e.:

Rods to 8 ft average spinning handle length12 inches?
8ft to 10 feet, 14? 16? inches
10 ft and above, 24? 28? inches

Categories and handle length...suggestions for either or both would be much appreciated.

A lot of what you are asking depends on how the person fishes. Some hold the rod under their armpit, some hold the rod in their hand with the rear grip supported by their forearm. There is no set formula, it all depends on the individual you are making the rod for.

Here is a general design plan that I just thought of in response to your question. Starting at 6' or less for spinniing rods, make the handle 10" total handle length. Add 2" for every foot increase in rod length up to 10' or more where I would stop at a total handle length of 18". More than 18" just seems cumbersome and in the way to me.

The type fishing being done is really going to determine that more than the length rod will.

.................

for most fishing I have found that handle length in excess of 10" tends to get caught up in clothing while fishing and casting 1 handed. Longest spinning rod I have is about 7-6.

You might be better off grabbing blanks of the chosen length and cast as one would as fishing
This way you can get an idea of how they are fished and how long the handles should be as they are fished ??
make sense

Bill - willierods.com

Jim, I just measured 3 7' glass trout rods I just put handles on, they are each 8" rear grip and 3" fore grip. Cork. A graphite 7' bass, rockfish rod has a 9" rear and 3.5" fore. I don't do split grips. Think they are ugly and do not look handsome on a rod. On salmon and steelhead rods, I measure the fore arm of the customer and make the rear grip to come to the point of the elbow. Not many people here in WA St tuck the rod under their arm pit. Longer rods, the same. Just made myself a mooching rod 11' long with a 15" rear grip and 4.5" fore. I have long arms. Like I said , a lot depends on the customer, type of rod he wants, and like Tom said the type of fish and water. Hope this helps.

Thanks Gentlemen.