Hi, MEZIERE and a few other EWP Manufacturers actually have an Idler with an identical diameter Water Pump idler Pulley mounted to them, so you may reinstall the belt as you normally would, using your original Belt.
This isn’t the first time this was done, the new belt length required is known & available online, may be included with the new electric water pump, it’s Mfg, or Parts vendor you purchase it from.I.e. Summit, A.Muscle, CJ Pony, etc..
I only had to measure out one, all my other Cars I’d previously shown that have idlers on them, the new belt length required was easily available through one of the above sources.
IF you HAD to, (unlikely will, but it’s easy). A crude, yet simple & accurate method is by mocking it up by measuring the required length using the original Belt, or an old one that you Cut & run around all the belt driven accessories, marking it with a Sharpie to calculate the length required, adding about 1/4-3/8” that the tensioner will take up, cut to correct length & Tape together with duct tape, once you’re happy with it, bring it to the parts store to match it up.
for the tensioner.. Tape it it so it is JUST able to
fit over the pulleys, once installing the new belt, the belt tensioner will tighten the slack.
This link shows a MEZIERE Electric Pump with an Idler installed, there are others like AEM that are cheaper & also have an idler, or the Belt # to be used based on your accessories, so you don’t have to mock it up to find the correct Belt size.. These are expensive, an AEM is a very, very good pump & Costs half as much, just for reference..
https://www.meziere.com/Products/Co...B-High-Flow-Street-Pump-55GPM-with-Idler.aspx
If you have any issues, add to this thread & I or another will help you get it straightened out..
Good luck! John