You will get the most out of it if you can combine four longhauls on one ticket. Like DEL-HEL-JFK-HEL-DEL. In J, that will give you 80 000 + positioning flights.

Ideally, you'd need to find AY tickets between the US and Japan. Whether AY sells those (and if they do, at what price?), I don't know. But if it works out, then you could buy one HEL-JFK-HEL and one JFK-HEL-NRT-HEL-JFK.

If you can't buy a J ticket from the US to Japan, consider splitting the whole thing into two, where you would visit Japan separately and then create a separate MR.

This year, I've bought MXP-HEL-NRT-HEL-FCO in J for about €2100. Substitute Italy with whatever is cheapest right now, and you'll earn 50k + positioning flights.

Then you'd need 50k from a MR. Why not make a similar trip to Japan if you have in-laws there etc? But basically, you could choose any longhaul destination ex-EUR, whatever is cheapest at the moment. If you have no intention to stay at the destination, then DEL is convenient and often cheap (and works even with the current, longer flight times).