Schengen days calculator

A non-EU citizen can only spend 90 days in ANY 180 day period within the EU Schengen area. Since the 180 days is a rolling time period, starting on the arrival day in a Schengen country, it is very hard to calculate how many days you can plan to stay if you make a number of visits on a frequent basis since the calculation has to be done forwards as well as backwards to determine what is allowed. Arrival and departure days are counted as in, even if for a minute.
I usually make 4 or 5 visits lasting 3-6 weeks each in any calendar year. Any help would be wonderful - and I am sure many others would value it too as the intention is to make this strictly enforceable by the end of 2023. All thanks to Brexit!