The boiler & radiation sizing is best matched for the heat loss in BTU/hour at the 1-3% coldest hours of the year, which will typically occur in 1-3 hour stretches in the pre-dawn hours of the coldest day of the year, never for 24 hour stretches.
To come up with an approximate annual fuel use number you need to know the BTU/hr at design condition, the average efficiency of the boiler, and the total number of heating degree-days (base 65) that you would experience in a year based on the weather history. As an example, let's use your 30,000BTU/hr as your design condition heat load, 90% as the average boiler efficiency, and use nearby Medford OR for design temp and weather history data.
According to table 1A on p17, your 99th percentile outside design temperature per ACCA Manual-J is +24F. Using 65F for an outside "balance" temperature base between heating & cooling loads, that's
(65 - 24=) 41F heating degrees, at which your load would be 30,000 BTU/hr.
But in a 90% boiler it would be burning (30,000/0.9 =) 33,333 BTU of source fuel or 0.33 therms per hour.
If it were to stay that cold for 24 hours that would indeed be (24 x 0.33=) 8 therms.
But that day would count for 41 heating degree days, so the boiler burns in fact (8/41) = 0.195 therms per heating degree-day.
According to ClimateZone your annual HDD/65F is about 4600 HDD so the amount of heating fuel you would burn in a year is (4600 x 0.195=) 897 therms/year.
The coldest average winter month is January, at 834 heating degree days, so a peak mid-winter monthly billing period would run about (834 x 0.195=) 163 therms/month, but less on average over the heating season.
A design heat load of 30,000 @ 24F would be slightly to the high side for a tight 1600' 2x4 house with an average glazing fraction and double pane windows, but is realistic at higher air infiltration rates. With almost any 1970s vintage home it's worth air-sealing and spot-insulating, and either adding storm windows (low-E storm windows are even more cost-effective with a quicker payback but cost more up front), or if there's no other way around it, replacing some windows with a U0.30 or lower window.
If yours is an open floor plan and you have lower than average electricity rates it may be slightly cheaper to heat with a ductless mini-split heat pump, which should have an average system coefficient of performance (COP) of 3.0-3.5 in your area. With an older R22 or R12 refrigerant heat pump with ducts the odds are you were only getting a COP of 2.0-2.5. Whether condensing gas v.s. split-system heat pump is cheaper depends on your local electricity and gas rates, and it may be a toss-up.