Ok, here is what the pilot's handbook says:
44. Cruising
(i) For any required airspeed, the maximum weak mixture boost (+7 lb./sq. in.) together with the lowest practicable r.p.m. provide the most economical conditions.
|(ii) When cruising at low r.p.m. the engines should be cleared every 30 mins. at +12 lb./sq. in. boost and 2,850 r.p.m. for 30 secs.
(iii)At any height the speed for maximum range is 170 knots at a weight of 17,000 lb. but below 6,000 ft. (my guess, the first digit is obscured, but looks most like a six to me) this speed can only be obtained and an uneconomical boost setting, even when using minimum r.p.m. Speed should therefore be increased to approximately 200 knots.
(iv) Fly with the supercharger gear change switch in the MOD position, unless the recommended airspeed cannot be obtained without exceeding 2,650 r.p.m., when high gear should be engaged by switching to AUTO.
45. Flight planning charts
The recommended cruising speeds ANMPG and GPH curves for a mean weight of 17,000 lb. and 20,000 lb. at sea level. 10,000 ft. and 20,000 ft. in low gear and at 25,000 ft. in high gear are on pages 30 to 33.
The charts that are on the following pages show fuel consumption at various RPM settings and list cruise speeds between 170 and 240 knots at sea level, 200 and 290 knots at 10,000ft, 210 and 300 knots at 20,000ft and 220 and 305 knots at 25,000ft.
On page 36 it lists the following limitations for the Merlin 25:
MAX. TAKE-OFF: low gear; 3000rpm; +18lbs boost
INTERMEDIATE 1 HR LIMIT: low/high gear; 2850rpm; +9lbs boost; 125 degree coolant; 90 degree oil
MAXIMUM CONTINUOUS: low/high gear; 2650rpm; +7lbs boost; 105(115) degree coolant; 90 degree oil
OPERATIONAL NECESSITY 5 MINS. LIMIT: low/high gear; 3,000rpm; +18lbs boost; 135 degree coolant; 105 degree oil
That is about all that I see in the pilots handbook that is relevant to cruising.