Float using Mandatory = 30 dy float for example, but "On or Before" same dates sais 0 - why? should be 30 also...

never ever evr use mandatory anything becsue it means just that. I think you have this reversed mandatorty will always say 0 and on or before will give you the corect float value.

This is becasue if the date calculated is ON or BFORE the constraint date the float wil be positive. If the date calculated is after the ON or BEFORE constraint date then the float will be negative.

There are 2 wasy to handle the completion date of a project. You can put a finish on or before constraint date on your last activity and or put a project musft finish by date on the project itself. 

If you do not use a project must finish by date and if you happen have a activity without successor then the late dates will be equal to the early dates and the float value will be incorrect. If you use a project must finish by date and are missing a successor the backward pass will start from the project must finish by constraint date and work backwards. 

I perfer use the project must finish by date so the backward pass knows where to begin and the float valure are correct. 

If you do not have a project must finish by date or a finish on or before constraint date on your last activity then there is no place to start the backward pass and you float will ALWASY be 0