What is goal? just to size the the house? Cost the system? Pay back on HVAC upgrade? Pay back sealing building? How much do you do a year? If just one a yr just MJ 5 or do it by hand. If like me 3-4 a day then need it quick I will use design star / write soft and this will give pay back and pre sell the job.
I like Wright soft with drawing boxes, I can run a load in 10 min whole house. room to room in 60 min. I find the number a little high 20% if not using blower door and right R- in walls and attic. if building is rated very loose and end up tight then very over in BTU's. I like to run the heating system at design temp and see if it runs 100% at night, that's my goal. Most systems are 300% to large around here I see a 120,000 BTU and needs only 33,000 BTU at very cold design temp or a 3 ton AC after attic sealed and windows redone just 2 ton is needed. Look at the tablet whole house app.
I got to agree with Eric on WrightSoft - with that let me also add; get training on it. It is to easy to get the numbers wrong & it will help you make sure you aren't oversizing to much. With that they will also show you that Manual J is not the end all be all - you still have to work through Manuals S, T, & D and how to get it right using their software.
Elite software is what I use. The company also has tech support.
Wrightsoft. With the accurate inputs, full version, I have hit the actual design load--clocked meter- within 3% and one, which deliberately under-sized, anticipating unfinished improvements proved within 5%.
The learning curve can be steep but we design radiant floors, walls, driveways and Unico high velocity as well as low velocity all with one brand.
how did you prove the load is right? This winter I am putting a hour meters on heating systems on a very tight buildings. I find 80% of systems are very over sized 3-4 times is very common
Clock the meter under design conditions.
do you find the gas gas meter to be off as much as 30% - more when old? Just Change out the meter then gas cost go up? I find the inside cups fill full of "tar" like a carburetor, and bearing get coated.