SWIM recently hydrolysed 1kg of glycol with 3L of 15% Hcl in a 5L flask for 5 hours and got 78% yeild.
Did you start with 1L of iso? And did you run the performic in a 5L flask as well? I thought the most you could hope for in a 5L flask was 750ml of iso, as per baal and labtop.
If you have the time, how about a complete writeup placebo?
It is also very tedious to remove all the excess formic acid by vacuum distillation (it cannot be distilled at ordinary pressure, or there is a risk of decomposition).