A spring rate of 390 and a load of 120 tells you the spring is shortened by 0.308 when installed. With a 1.9 height, that's a "free" length of 2.208 before installation.
Spring load is equal to original length minus final length times rate. Or R*(Li-Lf).
You can also add load plus the quantity change in length times rate. For example, if the load is already 120 and you add 0.100 more preload, you can do the following: 120 + (390*0.100), which would be 120+39, or 159 lbs.
Also, subtract the added preload from max lift. If max lift was 0.650 and 0.100 of preload is added, max lift is now 0.550.
Clear as mud?
