As for these calculators and royalty predictions spread out over the lifetime of a well, I would be very skeptical of anyone trying to make predictions without firm knowledge of lease terms and the highly unpredictable nature of the industry itself.
I agree about predicting future royalties....predicting well production is a crap shoot at best. Then, you have the price of NG, which renders any future predictions of income from royalties subject to the whims of the commodities market.
My Dad had good advice......don't spend your royalties until they are in your bank account. You would do well to factor in the decline of the well and therefore, royalty amounts, when considering how you might spend the money. Keep some aside for those rainy days.
Plus you gotta factor in the possibility of producers deducting post production costs from your royalties, even if you have a "no deductions" lease! Their take on the subject? "sue me".
My royalty statement shows unit production, price for oil, NGL and natural gas for the entire unit and then the same things for my percentage of the acreage we have in the unit. There are no deductions listed on my statement, which is about one page long, page and a half counting the attached check.