Hmmm. "Common wisdom" is not always right, but in this situation, common wisdom says that when the trailer is up on the tongue jack and level, the coupler will be about 19" above the pavement. This suggests that the unloaded ball height should be about 20 inches above the pavement, so that when you drop the weight of the coupler on it, and the rear end of the tow vehicle sags under the load, the TM will still be level.
Your easiest solution might be to drop over to your local WalMart. They have an RV section that usually has a good selection of 5000-pound Reese (good brand) ball mounts. Buy one, take it home, try it out - and then return it for another if the TM is not level when hooked up. Note that these simple ball mounts can be installed upside down, so that in addition to providing drop, they can also provide rise if needed.
Bill
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