In conventional houses the heat load increases approximately linearly with outdoor temps by the number of degrees below 65F. At 65F there's no heat load, and the output temp of the boiler doesn't really matter. But you can use fuel use against heating degree-day (HDD) data to determine the whole-house heat load at any arbitrary temperature, including whatever you outdoor design temperature (in Seattle the design temps are around +20F.) Get a gas bill, look at the precise meter-reading dates, then go to degreedays.net and download a spreadsheet of base-65F degree-days for a weather station near you, that covers the entire billing period, add up the HDD between the billing dates, and do the simple math of therms (or CCF)/HDD. 1 therm=100,000BTU, and for our purposes, a CCF is close enough to a therm to use the same numbers.

For a mod-con, assume you'll be getting close to 90% no matter what, so once you have therms/HDD, multiply by 0.9 to come up with BTU/HDD. Then divide by 24 to come up with BTU/degree-hour.

Now you have something by which you can estimate the load at any temperature: At 25F, you are 40 degrees below 65F (your presumed heating/cooling balance point), and your whole house heat load is 40 times your BTU/degree-hour number. At 0F you have 65 heating degrees, so the heat load would be about 65 x your BTU/deg-hr number.

To set the curve you then have to know how much heat your radiation will put out at given water temperature with your given number of feet. Pick a number close to your design-temperature (say 20F) the calculate your BTU/hr heat load number at that temp. Then looking at the specs for your radiation, given the number of feet you have, figure out the water temp that's necessary to deliver the heat at that rate. For the sake of argument, lets assume your calculated heat load at 20F was 18,000 BTU/hour and you were running a total of 40 feet of radiant baseboard rated at:

580BTU/ft @ 180F

320BTU/ft @ 140F

160BTU/ft @ 110F

With 40' of radiation needs to be able to put out 18,000/40= 450BTU/ft to keep up, which is about half-way between the 140F and 180F output numbers, so the water temp would have to be about halfway between 140F and 180F, or ~160F for the radiation to keep up when it's 20F outside, so you would set the bottom of your curve to deliver 160F @ 20F outdoor temps.

Then calculate what your heat load would be at some significantly warmer temp. If it's 18,000BTU/hr @ 20F (a 45F delta below 65), that's ~400BTU/hr per degree below 65F. So at say 55F, it would need to deliver about 400 x (65F-55F)= 4000 BTU, or 100 BTU/ft. Most baseboard is somewhat non-linear below 110F, but you can assume that with 90-95F water would still be delivering at least 400 BTU/ft, or you could do a linear interpolation from the 140F and 110F numbers to estimate it.

Another approach for picking the higher outdoor temp of the curve would be to use the 110F output number to determine the outdoor temp at which would need that. With 40' x 160BTU/ft= 6400 BTU/hr, that's good for 6400/400= 16F below 65F, or about 49-50F for an outdoor temp. Similarly, calculate the outdoor temp at which the radiation balances with the heat load with 180F water, and set the other point on the curve there: 40' x 580BTU/ft= 23,200BTU/hr. 23,000/400BTU per degree is 58F below 65, which is +7F.

Using those as starting points for defining the reset curve, if it's keeping up when it's cold outside, but doesn't when it's fairly warm out, but up the water temp at the 50F outdoor point. If it keeps up when it's mild out but not when it's colder, bump up the water temp at the 20F point of the curve. If it's keep up all the time, start backing off the water temps at both points on the reset curve ~5F at a time until it starts having issues, then increase them a degree or two at a time until it's back working for you. At that point you'll be getting about the best-possible efficiency out of the system.

If the radiation was sized reasonably on a room-by-room basis by the original designer, the heat loads and temperatures room-to-room and zone-to-zone will pretty much track, but don't be surprised if one zone keeps up a slightly lower temp than the other. It's up to you to decide if tolerating a slightly cooler zone or room is worth the extra percent or two in efficiency, but hopefully it'll be close enough to not matter very much.