What's the math of spring bars sizes?

What are you towing with? Reason I ask is I had a similar size TT and towed it with two different trucks. One was a 2010 F150 Max Tow 154" WB, and the other was a 2102 Ram 2500 diesel CC LB 169" WB. The F150 need all the 1200 bars could muster to transfer enough weight back to the front. We also had a fair amount of stuff in the trucks bed.
With the 2500 and the heavy Cummins sitting up front I didn't need to return hardly any weight to return the front to normal height. I didn't need much of the 1200 bars. Could've went with light if needed. Sold the TT and got a 5th wheel so never had to deal with it.
Point being is there's variables that come into play when choosing bars.

My trailer came as a package deal with 400-600 lb bars because the listed dry weight of the trailer was 4370 lbs. The dealer clearly only wanted to make the sale, thought I would be fine with my 5200 lb tow limit SUV at the time and knew my maximum tongue weight rating with WDH on the SUV was 600 lbs. Actual tongue weight of fully loaded 6006 lb trailer is 878 lbs. I purchased 1200 lb bars and have noticed that I don't have to put too much tension / bend on them at all. I'm using links 3 or 4 where previously I was using 5 or 6 and really bending the bars.

Ride is much better now, a lot less trailer bounce over bumps and dips.

Using your guesses on weight, it does appear that 1200 would be the best option. However, 1000# would work as well.

The difference in ride between the two will be noticeable. The 1200 bars will tend give a stiffer ride when hitting bumps, while the 1000 bars will flex more and cushion more.

Your question is what is the math for spring bars. Your statement is that you do not know the actual toung weight. So fill in the missing blank by getting the acdtual TW then buy the bars that are appropriately sized.

You need the true tongue weight while fully loaded and ready to travel.

If that is unavailable, buy bars rated for 15% of the trailer's GVWR.

If your GVWR is 8,750, the correct bar would be rated 1,275.

Contact the hitch manufacturer as to whether the 1,000 or 1,400 bars would be appropriate. I'm surprised they don't have a 1,200 pound option.

The size of the bars are for how much weight you want to transfer. Until your tongue weight exceeds the amount of weight you need to transfer, your fine. Bigger bars cannot transfer weight that is not there. At 875 pounds, you still have 125 pounds to transfer, if you are indeed transferring 100% of your tongue weight. Get on a scale, weigh the steer axle without the bars attached, then weigh with them attached. The difference is how much weight you are transferring. When you cannot get your rig to level out and are still light in the steer axle, then you need to adjust the head. Most of the time, a simple head adjustment is all that is really needed. If you go bigger bars and don't have the weight for them, you will have a harsh ride.

I agree that 1200 pound bars would be appropriate. I would take a trip to a scale before spending money on different ones though. Your truck may not need 100% front axle weight restored so you may be ok with the current bars.