We have a similar sized house.
Filling the tank costs ***about*** 1k (ranges between 800 & 1300 depending on the price per gallon). We have to pay 40% of our bill each month.
So 40% of 1000 = $400 in Sept
40% of 600 = $240 Oct
40% of 360 = $144 Nov
40% of 216 = $86 Dec
TANK REFILL OF 1000
40% of ....
Now... that's J. 'theory'. In actuality, we never pay less than $200 a month so that we can stay caught up/ it's not so painful whenever the tank gets refilled.
We usually have to refill twice in the winter, and then the second tank lasts all spring/summer and we get a refill in the fall.
We've found it MUCH less expensive to avoid space heaters (saves us about $100 per month, but only gives us an extra few weeks to month's worth of oil... so we're saving $300-$400 by not using them). The oil heats up the whole house very, very quickly. (From 50-75 in about 20 minutes). Keeping things an ambient of about 70 DAY AND NIGHT (there or not) saves us the most. We've tried turning the heat down/off at night... keeping it consistantly hotter, and keeping it colder. About 70 24/7 means our furnace clicks on 4-5 times a day (24 hour period) for 2-3 minutes each time. 10-15 minutes of burn time per day. Turning the heat down at night, the furnace will blow for about 20 minutes in the morning, and then another 4-5 times a day for 2-3 minutes. Meaning 30-35 minutes of burn a day. Ditto, keeping the heat higher increases the burn frequency, using up more oil.
Whenever I cook, I leave the oven open when I'm done (so it heats the house instead of dissipating up the vents), and I keep the fans off as much as possible (kitchen and bathroom)
I ALSO keep an eye on oil prices. Even if we only have 50-100 gallons of free space in the tank... if oil prices drop I schedule a delivery.
I'd FAR rather pay $2.16 per gallon than $4.27. Keeping the tank filled with the lowest price possible is common sense.
ALSO Change your air filters at least every 6mo. (to improve efficiency) If I have a dirty air filter, that 2-3 minute burn turns into a 5-6 minute burn. AKA I'm eating twice as much oil. It is the CHEAPEST thing to do to increase efficiency. ($20). If you're strapped and can't afford $20... take the durn thing upstairs and hose it off and let it dry.