I was trying to figure out the grains of powder I would need to use for my reload of 50bmg.
I am using H50BMG.  I talked to Hodgdon and asked if they could help since their website did not have a weight for my bullet.  The said they did not have the facilties to test there and that their information came from Australia.  He suggested that with my bullet weight of 690 grns that I should start with 225 grns of H50BMG.  I started looking at their numbers and came up with this result.
 I took the bullet wts.(see chart below) 750 - 655 = 95 then powder wts. 248-233 =15.  
Then 15 / 95 =.1578947.  I rounded this out to 2 decimal places and came up with
.16 grains of powder per 1 grain of bullet weight.  
To test this result I took the 655-800=145 then 145 x .16 = 23.2.
248 - 23.2 = 224.8, which when rounded up is 225.  This matched what is shown on their charts below. I applied this to my bullet weight.  690 - 655 = 35,  35 x .16 = 5.6.
248 - 5.6 = 242.2.  I then took this number, which is max powder charge and reduced by 10%.   242.2 x .9 = 218.16.  According to this I would start my loads at 218 grains of H50BMG for my 690 grain bullet.

Now I am a newbie at this.  I have loaded .223 and .308, but this will be my first time doing 50 bmg.  I would like some input from the reloaders out there experience in this to tell me if this is wacko or do you think I am close to being right.

Thanks for any help!

THe listing is a max load.  I would start out with about 220gr and work up from there.
Chrono each load and watch the cases and primers for overpressure signs.

There are a few other factors to be looked at when extrapolating information such as loads of powder for a unknown bullet.

Is it lighter or heavier than bullet "X"? How will this change the volume of the case (bigger bullets sit deeper in a case than a lighter bullet). Some bullets that weigh the same might have a different bearing surface, thus more drag or friction in the bore. example is a flat base versus a boat tail. With the flat base having more bearing surface rubbing against the bore.

I agree with rbthntr64, you need to chrono the round and look for pressure signs. And the golden rule is to start at least 10% below. But I would start 10% below starting amount.

Loading saves money, is relaxing to many, supplies pride in your work when you fire one of your handloads in the bullseye. And you can tune the ammo to get the best accuracy out of your rifle. But be safe and careful of what you are doing. Pistol loaders have loaded double charges. I once forgot the power but did put in a new primer for my .223. But was about 1" from muzzle and the brass did extract and loaded a new round into the chamber. But I was attentive to the fact that the recoil and report was all wrong. I hate to have pulled the trigger again and maybe blown my rifle up. So, be careful.