I would suggest stacking up on 10ks if possible (be sure to get them in an incubator AFTER the event starts or they could get scrambled up and not benefit from it) and then using the infinite one exclusively on 2ks if you have a starpiece running. Without a starpiece I would break as many new 10ks or 5ks as possible in the infinite incubator. 2ks give the most stardust per km and since there will be no added bonus from walking 5 or 10ks (no extra stardust or candies) I think it would be best to save any new ones for a later opportunity.
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Community day: Walking
Reducing the egg distance to 1/4th for community day is an interesting choice. But how to best take advantage of it? If you walk for the complete three hours with a normal speed of 4 km/h, you will complete a total of 12 km. With only your permanent incubator that is 4 of 10 km eggs or 9 of 5 km eggs or a whopping 24 of 2 km eggs (if you find enough new ones).
I guess, saving up as many 10 km eggs as possible and enough incubators is the thing to do for preparation. That, and finding a nice walking circuit.
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Good question, as a f2p player I see 10km eggs and incubators soon most collectable again. Those with endless money for the game won't have much to spend on but super incubators.
I can't see Ampharos getting more powerful moves than Raikou already possesses so... just another glorified grind for most.
It just occurred to me that Niantic is really doing a fine job keeping us on our toes here: On 15th April you'll want as many 10 km eggs as possible in your box. But nobody is getting anything but 2 km eggs until 2nd April. Which has been the case since 22nd March, just a few days after the end of Windy Egg event that prompted many players to clean out their boxes.
Using incubators during reduced hatching distance is actually not a good idea. What you're buying with incubators is walking distance that you don't actually have to walk to hatch that egg. With 1/4th hatching distance, you only get 1/4th what you would normally pay for. There is no real advantage in hatching as many eggs as possible during the 3h window, the bonus just means your infinite incubator becomes much better.
So personally I'll check myself a route that optimizes distance taking into account the update interval the game uses, restart at event start to reset the timer and start walking with the infinite incubator. Prioritizing hatching 10ks and 5ks as 2ks might get a lot of wasted distance if you walk more than 500m in 8 minutes.
