One way would be to buy a box of THAT ammo and take one or two apart and try to identify the powder & weigh it.
Aside from that, start with max loads for a Marlin lever gun in .45/70, go from there, if pressure signs indicate you can. You will not find the data you seek from any powder company or a loading manual, unless you can find an old AA(Accurate Arms) manual with the red covers. They had the data you are looking for. The top loads for a Marlin .45/70 or .458 Marlin are likely 40,000psi to not over 43,000psi. This 43,000psi is the maximum suggested for the Marlins. The 86 WILL 'take" a bit more. These cases are immensely strong in the head area due to the rim diameter(mostly). Marlin noted some time ago, they loaded a case to 70,000psi and had no problems with the rifle or the case. 70,000 is not loading data, but they did that to show what the rifle with it's small barrel diameter and the ctg. would 'stand'.