I was wondering how much I need to shorten a 0.370 diameter bullet to reduce the weight by 25 grains but I have no idea what lead weights.     Tom

Tom the best way to do it is take one of those bullets and start filing it off the back end until you reach the final weight that you want. That will provide with a more exacting length and what you need to shorten your bullet mould too.

cut and weigh to be sure   art

What is the weight per cubic inch of 30:1?

The specific gravity of bullet alloy, or any other solid, is easily done. A scale/beam balance, some water, a cup, thread and a sample will do it.
The pictures won't work; for a copy, ask
joeb33050@yahoo.com

BULLET ALLOY SPECIFIC GRAVITY

     Cast bullet shooters are sometimes interested in the composition and hardness of bullet alloys. Precise assays of these alloys are expensive, and individual shooters seldom want to know enough to pay for the test. Hardness of bullet alloys can be measured with hardness testers that are available from LBT, Saeco and Lee. 
     I don't have a hardness tester, and have used the Specific Gravity of alloys to estimate the composition and hardness of those alloys. This isn't precise, but it's close enough for me.
     My records of weights of 311299 bullets shows the weight varying from an average of 208.8 grains (wheelweights) down to 197.8 grains (wheelweights with foundry type added). This is a difference of about 5% in weight and specific gravity, and this difference of 5% is a great difference in percentage of tin or antimony and in hardness. Keeping track of weights of bullets in different alloys is one way to estimate the hardness of the alloy. Measuring the specific gravity is another.      
     The Specific Gravity of a material is the ratio of that material's density to the density of water. If a quart of material K weighs twice as much as a quart of water, the Specific Gravity of material K is 2. If a cubic inch of metal L weighs eleven times as much as a cubic inch of water, the Specific Gravity of metal L is 11. For lead-tin-antimony alloys, as the specific gravity goes down the hardness goes up.  If there are other metals such as gold or silver or zinc or arsenic or cadmium or copper, we'll never know by calculating the Specific Gravity; but if it casts good bullets it probably doesn't have much of anything exotic in the alloy.
Specific Gravity
Weight of sample (bullet) in air
minus   Weight of sample in water
equals   Weight of the water displaced by the sample 

Weight of sample (bullet) in air
divided by      Weight of the water displaced by the sample 
equals            Specific Gravity of the sample 

     To measure the Specific gravity of an alloy we need a sample of the alloy-a bullet works fine, a scale, a glass, some water and a piece of thread.
     Here's the scale up in the air with a bullet suspended by a thread from the pan holder. Just hanging there. The bullet with thread weighs 434.5 grains.










Here's the bullet in a glass of water, not touching the sides or bottom of the glass. Just hanging there. The bullet and thread weigh 395.6 grains in the water. 
     We can safely forget the weight and volume of the thread.


Weight of sample (bullet) in air                  434.5 grains
     minus   
Weight of sample in water                        395.6 grains
     equals   
Weight of the water displaced by the sample      38.9 grains (Eureka!)

Weight of sample (bullet) in air                  434.5 grains
     divided by      
Weight of the water displaced by the sample      38.9 grains
     equals            
Specific Gravity of the sample                   11.17      

Tom hopefully you already have it done by now and if so how did the end result turn out for you.

fair enough. hope it shoots good when you get it.