Christian has the correct answer for sulfur loading.
And to check your loading, just do elemental analysis. If you have any deeply trapped sulfur in the pores, TGA would give an underestimation of sulfur content.
Apart from the standard approach mentioned above, You can also use Lithium Polysulfide solution to introduce sulfur. The advantage with this approach being that there is good electrical contact of sulfur species with electrode and electrolyte.
Lithium Polysulfide can also be introduced as a solid as in the full cell paper
You can achieve 70% loading by appropriately adjusting molarity of solution and volume of solution introduced in the cell.