Zarg, there are two ways you can do this. One is at the start of your song write in a CC1 127, or any other number for for your volume. 127 is the loudest but you could put 100, 91, 55 etc. Just set the volume to what you want. You can do this in the PRV by clicking on the little + sign on the velocity track. It will ask you for a CC number and put in 1 then draw in your volume. Do this before any notes. It the notes start on measure on beat one then add blank a measure before your song. It is a good idea to do this to any song, add a blank measure that is.
The second way would be at the start of your song before and notes record your mod wheel, the mod wheel is CC1. Again move it to the volume you want. If you over or under shoot your volume aim then just edit it in the PRV.
I hope this helps.