Coolant amount in 99 WS 3.8L`

I drained the coolant through the drain olug, but it was oly 1.5 gals (6 qts) rather than abbout 3.7 gals. If I add about 1.5 gals of 50/50 antifreeze, the actual percentage of antefreeze will be only 20-25%. How did you drain all the coolant? If you were not able to, how did you make actual mix ratio of 50/50?
Prompt replys will be greatly appreciated since my car is on jacks now.

Assuming you want to get rid of most of the old coolant. Perhaps the easiest way is to keep adding fresh water and draining ... do this repeatedly until the drained liquid is almost free of color.

Then on the refill, add pure anti-freeze ... testing with a tester, looking for -45deg F freeze point. Adjust ratio as system nears being full.

If you do not have the rear heater.....for the 1996 3.8L (I'll bet yours is the same), the capacity is 12.1 quarts.
SO.....if you do the water method (use distilled water), you would add at least 6 quarts of pure coolant to get a 50/50 mix.
If a slight bit short......fill the overflow bottle with 100%.

Now, burping all the air out is important....
It is not uncommon for some air to get trapped in the system.......and your temperature to shoot up.
When this happens, shut the motor down and let it cool.
Once cool....make sure the system is full, not just the overflow bottle, but the upper radiator hose should be full of coolant.....you can tell be squeezing it.