I tried this once many years ago when I used DDX and found that 50% greater times than 1+4 were OK but the negatives were a little thin and required about half a grade to a grade extra when printing. Others said they tried 1+9 and found that between 60-70% more was about right
Try it. Based on my experience the negatives were OK and printable but thin and I think you will safe if you use at least 60% when the negatives will be closer to what you would get at 1+4 tímes
Ilford itself makes no mention of 1+9 but it does give a method of using stock repeatedly and increasing the times. Have a look at its information sheet
If you are a hybrid printer, using a scanner, Photoshop and inket printer then what you can do with PS may mean that 1+9 gives you everything that you need
It all depends on what your standards in terms of the prints
pentaxuser