Nope.
1gram of 65% BY WEIGHT would be 650mg.
If potency is given as % by volume, then you need a known volume, not a weight.
If the density of your concentrate was exactly 1.00g/ml your math would give the correct result despite being malformed…
… then you tell us there are 10,000mg in a gram!!! (There are 1000!).
And that 10% of that 10,000mg is 100mg.
While you have solved for the number of mg of thc in 1g of 10% flower, you have most certainly not helped anyone else understand the problem.
Using “volume of a gram” rather than “a gram” muddies the waters further…
Edit: for folks reading along at home; try over here if you need help with formulation Need formula to dose 10mg THC capsules 🥄