Old Rules of Thumb
Rule of Thumb down in the lower reaches of the atmosphere is that you lose about 1 lb/sq. inch of atmosphere pressure for each thousand feet of climb, therefore if you could get 29" Manifold Pressure at Sea Level (for example), then at 2000' above SL you would have a max of 27" MP available.
This needs to take density altitude into account. How?
For each degree C above standard temp for the altitude, add 120 feet to the pressure altitude. This will give you Density Altitude.
Example: You are on an airfield, dial in 29.92 on your altimeter and it reads 1000 feet. This is Pressure altitude (close enough for your purposes). The Temperature is +21C. Standard Temp for 1000 ft PA is +13C. That means 8 degrees C above standard. 8 x 120 = 960...about 1000...so add 1000 feet to the pressure altitude of 1000 feet and you get a Density Altitude of 2000 feet.
Now using the Rule of Thumb above, if your mill would develop 29" MP at Sea Level, you can expect it won't do more than 27" MP where you are at in the example.
Now, Horsepower? If the book tells you the mill develops 150 HP pulling 29" of MP, then 27/29 = 139 HP, tops. In other words, about an 8% reduction in power...so you better get the ship about 8% below Max Gross, eh? For a hypothetical GW limit of 2000 lbs, this would mean offloading down to a TO weight of 1840 lbs. So there goes a lot of people or fuel...
But wait, there's more! Don't forget that Density Altitude of 2000 feet means less lift (less dense air...) being produced by the rotor blades (and less tail rotor thrust being produced, but we aren't concerned about that just this minute...). So less lift the blades can produce, means you better offload some more weight, eh? How much? Without actually doing a Lift Equation calculation, why not figure another 8%? So now your 2000 lb aircraft can only go with about a 1680 lb takeoff weight. A reduction of some 320 pounds...that's at least a pax and a half, or about 50 gallons of fuel...
Be advised, all the above is hypothetical and based on rules of thumb. The Rotorcraft Flight Manual is, in all cases, your best friend and will give you accurate information. But now you can understand why most folks opt for a turbocharged engine, if available.
A lot of manufacturer's data for these calculation is available online and you should visit the websites. In any and all cases, a call to a friendly sales rep will get you assistance and accurate planning figures.
Good luck.