I printed some test negatives as platinum prints and had a friend read the maximum density of the blacks. D Max topped out at 1.3. Is this as good as one can get with a platinum print given the materials I use? If not, what changes would I have to make to get a deeper blacks?

For an 8 x 10 print, the drop count is as follows:
12 drops Ferric Oxalate
10 drops Ferric Oxalate with Potassium chlorate
8 drops Potassium Chloroplatinate
16 drops Sodium Chloropallidate
Paper is Cranes Platinotype
Developer is Ammonium Citrate

If you are a devotee of pt/pd, then you know these are all from Bostick & Sullivan.