Theoretical writes/sec limit If I have solarwinds which has an alert set whenever we have more than 300 writes per second, and I know 300 writes per second is fine, and the writes per second often goes over 6500, how can I work out what the theoretical limit of writes per second is, so I can alter the alert thus making it actionable?

If "The number of writes should not exceed 85% of the disk capacity" how can I work out the number of writes that are possible with my current disk configuration?
One way to do this will be to monitor the writes/sec for weeks or a month and then come up with a number based on what the max values that you are seeing.  Obviously this is a rough guide, and as your workload changes you would have to adjust the threshold as well.

You could also use disk benchmarking tools to test the throughput of your disk subsystem.  diskspd is something which we use.  Please also note that writes/sec is dependant on the block size that is used.  Based on what the drive is used for(data or transaction log)  and also based on the type of workload, SQL Server could use different.  You can read about SQL IO sizes here.  Obviously if you run this on a production system, you might have a performance impact.

A better performance counter to alert on would be sec/Write or sec/Read.  You can read about it here and here