Auto-generated sequential value upon field activation

I am looking to generate a seuqential value in a request when a particular field is set, so in the attached image when the RMA Required is set to yes the RMA Number field is populated with the next incremental number in sequence (i.e. the last RMA number generated was 99, I would expect the field to be populated with 100).

Is there anyway to achieve this in SCP, or can I use API to connect to the database and execute an SQL script to generate the value and then use JavaScript to then set the field value accordingly.

This cannot be the first time this has been raised and I am quite surprised that this functionality is not more easily configurable than it is?

Thanks.