Ilford recommend you make test exposures first when using the film at 12500 and above. (See wwww.ilford.com) Also, For such high meter settings, they only give times for a few developers.(posted 9224 days ago)The recommended developpers are:
ILFORD DD-X (1+4) 17 --- 12 MICROPHEN (stock) 16 1/2 --- 13 1/2
Time are also given for:
ID-11 (stock) 17 --- 13 1/2 Kodak D-76 (stock) as for ID-11 Kodak T-Max (1+4) 14 --- 13 Kodak Xtol (stock) 12 1/2 --- 11
BTW: They also give some data for EI 25000/45:
ILFORD DD-X (1+4) 25 --- 17 MICROPHEN (stock) 22 --- 17 1/2
Key: Developer (dilution) time, in minutes, at 200C/680F --- time, in minutes, at 240C/750F