To get a really decent trajectory for 150 with a short barrel limits you to be limited to 100gr loads adds to that limitation since you have a 1-20 twist which will usually shoot longer bullets best also enters into the picture.
What the answer my friend came up with was 50/40 blue sabot Shock Wave 200gr [this is a 40 caliber bullet in a 50 caliber barrel] it give both the sectional density and BC while still having a enough bullet lenght that it should shoot in your 1-20 twist it does quite well in his.
In case your interestedthis is what I worked up for him when I was shooting with him.
!800fps 90 gr Proydex 50/40, 200gr SW[Chrony average]
"0" @ 100yds
125yds -3inch 150 yds -7.25 [average]

Since he is shooting the load that groups best in his gun and its not max you might want to work with the load before you try to calulate any trajectory. Lee