I get a film (Delta 100) clearing time of around 40 seconds in fresh Ilford Rapid Fixer 1+4.

The Ilford data sheets says it can be used either 1+4 or 1+9 for manual application - which of course means it can be used at in-between dilutions, too. So if I were you, I would dilute your unknown concentration 1+1.5 with water (in other words, if you have 1 litre of mixed fixer then add another 1.5 litres of water). This means the actual concentration will be between 1+4 (if it was originally 1+1) and 1+9 (if it was originally 1+3). Then I would do a clip test, and use it with a fixing time of at least twice the clearing time found in the clip test (you can split this total time equally between the two baths).