Ive fire formed 7.62x39 to 6.5 Grendel but you have to use less powder because the fire formed x39 cases have less volume than true 6.5 Grendel brass. When I fire formed my 7.62x39 brass to 6.5 Grendel I used the minimum powder charge listed in the manual for the cheapest bullet I had available. I had bought factory seconds Sierra 100 grain varmint bullets and I think some Winchester 748. Once fire formed I used more powder to get closer to the same velocity as loads in 6.5 Grendel brass.
You could weigh water in a non fire formed brass case compared to an actual 6.5 Grendel case filled with water to get the percentage difference between the two cases. You might then have a better idea of how much powder you could use. I never had any issues using the minimum starting charge in fire forming x39 brass to 6.5 Grendel. The Hornady 8 or 9th edition reloading manual used a 14.5 AR-15 barrel for developing the load data. The data for the 95 grain V-Max should give you an idea of the velocity of a 16 6.5 Grendel (16 might be 30-50 FPS higher than 14.5). The fire formed x39 brass can be loaded to a max pressure of 52,000 PSI just like 6.5 Grendel brass but your FF cases just get to that pressure with less powder.
My fire formed brass only lasted for a few reloads before the primer pockets got loose. Some x39 brass is easier to size than other brands. I used Dillon case lube with Hornady dies and ruined a couple pieces of x39 brass trying to size to 6.5 Grendel. If I remember correctly I had the most ruined cases with IMI brass.
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You could weigh water in a non fire formed brass case compared to an actual 6.5 Grendel case filled with water to get the percentage difference between the two cases. You might then have a better idea of how much powder you could use. I never had any issues using the minimum starting charge in fire forming x39 brass to 6.5 Grendel. The Hornady 8 or 9th edition reloading manual used a 14.5 AR-15 barrel for developing the load data. The data for the 95 grain V-Max should give you an idea of the velocity of a 16 6.5 Grendel (16 might be 30-50 FPS higher than 14.5). The fire formed x39 brass can be loaded to a max pressure of 52,000 PSI just like 6.5 Grendel brass but your FF cases just get to that pressure with less powder.
My fire formed brass only lasted for a few reloads before the primer pockets got loose. Some x39 brass is easier to size than other brands. I used Dillon case lube with Hornady dies and ruined a couple pieces of x39 brass trying to size to 6.5 Grendel. If I remember correctly I had the most ruined cases with IMI brass.