Yes you set your timing the correct way. Total timing is your initial timing + whatever the preset mechanical timing is in the distributor. The vacuum advance is NOT added to these figures.
In your case if you had a built in mechanical advance of 20 degrees and you added that to your initial of 15 degrees (with the vacuum advance hose disconnected and plugged) you would have a total of 35 degrees.
The mechanical advance weights combined with their return springs will determine WHEN the mechanical advance begins to come into play and at what RPM the weights are fully extended. You should have the full mechanical advance in somewhere between 2500 and 3000 RPM for most applications.