Gareth. I think you really need to take a workshop on digital negative making and alt-process printing. This will answer a lot of your questions related to density, exposing, Salt (that is a world un-too itself), etc.
For example, 1.7 is way way less density than every Salt print needs. Salt needs 3+ optical density of the neg (probably around 3.7 to 4.0 ideal UV transmission density). If the idea of transmission density is new to you, please read up on dig-neg and alt-proc methods (especially salt!).
For figuring out exposure, get a Stouffer strip: http://www.stouffer.net/using21step.htm
The formula for figuring out minimum time for maximum black is printed right on the packaging.
Depending upon your exposing unit (there are lots) you may be counting seconds (after your bulb warms up!) or maybe units (for platemakers, etc) or just winging it out-doors with a UV meter. There are many variables that you can learn that this software/manual and forum aren’t necessarily set up to teach and support. (We can certainly help with debugging exposure units though!)
The master curves for PiezoDN will make decent salt print negs. They will need to be linearized though (this will increase dMax) as this is a printed-out-process.