The best tool for those designing size-inclusive garments are spreadsheets (or tech editors). If you can afford a tech editor, there are many amazing and experienced ones out there. Even with the information that I give you, I still highly recommend finding yourself someone to tech edit your patterns to further ensure that your customers and testers get the best product possible. But, if you are just starting out, you most likely don't have the funds to pay for added services like grading and yardage calculations. Or, maybe you are embarrassed sending a super raw pattern to an editor. Whatever the case may be, you are here, and I just need to get to the point.
The point is, spreadsheets like Excel and Google Sheets (I use Google Sheets because it's free and automatically backs up) are excellent tools for designers and tech editors alike. So, here are three spreadsheet functions that will make your life easier:
This function will round the answer to the nearest fourth. If you change the ".25" to ".5" it will round it to the nearest half. This is super helpful when you are trying to get measurements in inches and centimeters. I like to round to the nearest fourth when I'm working with inches and the nearest half when I'm working with centimeters. Any measurements smaller than that are just a pain for you and the crafter.
=EVEN(function) / = ODD(function)
This will allow you to round to the nearest even or odd number. Being able to do this is super handy if you know that you need to have an odd or even number of stitches in order for your pattern to pan out.
=floor(function) / =ceiling(function)
These functions determine if you round to the lower integer or upper integer. So, if your calculations produced a 4.679, floor(4.679) = 4, and ceiling(4.679) = 5. I like to use the ceiling function when I am calculating yardage and the amount of skeins of suggested yarn that people will need for the pattern. People can't really buy 4.679 skeins of yarn, and 4 skeins of yarn would be too little. So, I have the spreadsheet do the work for me and give me the ceiling integer.
Hopefully you found this helpful! These functions save a lot of time, but be careful and make sure that your use of them doesn't end up changing the dimensions of the garment by a lot. It's always a good idea to go back and make sure that your math "adds up" lol.