I know that using inches is "easier" if you have not gotten involved with using metric measurements - but I'd like to suggest not making the change and leaving your measurements as metric so you can learn and have an easier time in the future.
Here's why-- If you are having to move designs around, re-center them, enlarge/reduce size etc... If you are using inch measurements - it's harder to determine what 1/2 would be if you are using 7/8" or 3/8" or 1-5/8" etc. These types of measurements are harder to cut in half and then you end up with 8ths, 16ths, 32ndths of an inch etc. But- if you are using metric and remember that 1in = 25mm, 1/2in = 12.5mm, 1/4in = 6mm, 1/8in = 3mm those 4 are the basis for being able to do most calculations using metrics. They are the ones you would use the most with quilting and embroidery. And once you start doing it pretty often, it comes naturally. So- 7/8in = 21-21.5mm and I typically drop that .5mm because it's really *so* small. (I added 12.5 + 6 + 3 = 21.5mm). And- if it's a problem for you - you can make a chart and keep it handy so it's even easier. But- here's the big thought... It's much easier to find the center of something that measures 21mm than it is 7/8" since you only cut 21 in half and come up with 10.5 (I round it to 10) so you can move something 10mm easier than you can stop to figure out what half of 7/8in is and then it's going to be off more when/if you round the # you get. And- if you are like me, when I was doing inches... I would have to have a tape measure and then use it to determine where half would be. Where's the tape measure when you need it?? To move a design 3/4in to the left (example) you would move it 18mm (1/2=12, 1/4=6mm).
Most digitizers use metrics for design sizes. Some will list them with both measurements because they never know who might be buying their designs and don't want anyone to have to do the math (altho it's not hard for many....) Most hoop sizes are listed in metric measurements so you can start from there. If your hoop is 200x200 then you know center is 100mm going from the notches on each side of your hoop. That's 4inches since you know that 25mm = 1in and 100mm divided by 25 is going to be 4. But if your hoop is 5.5in x 7.9in (like one of mine) .... it's harder to figure out where center is by cutting those numbers in half than if you use 140mm x 197mm. Cut both of the latter #s in half. Newer Janome larger hoops that are 9.1in x 11in convert to 227mm x 275mm (the 12000, 15000, 14000 models machines) or the 550e has the 7.9 x 14in which converts to 197 x 350mm.
Obviously, it's your choice. This is just something to think about and my opinion... If you are moving designs around to group them, ungroup then center etc.... Metric makes it easier. I really wish we had all grown up doing the metric system. I didn't but have converted most of my measurements since becoming a nurse 40yrs ago, quilter 30yrs ago, and machine embroiderer 20yrs ago.
Tracy in Nashville, TN
Martha Pullen Licensed Educator