A 158 gr bullet is 5.3% heavier than the 150 gr. Multiplying 1.8 gr x 1.053 = 1.896 or appox. 1.9 gr, which is 0.1gr under the 2.0gr max for the 150 gr bullet. With the heavier bullet you could drop down to 1.7 gr. But it may not make that much difference depending on the guns you are shooting. Keep in mind, however, that other factors may obtain, such as bullet diameter, hardness, etc. One needs to be a little careful with minimum loads with Trailboss, especially in cold weather. I would try shooting at least 10 rounds with 1.7 gr and 1.8 gr with the 158 gr. bullet and see if you get changes in accuracy, Point of Impact, etc.