Going slower will use less fuel and take longer, you are exactly right.

The short version is you take your acceleration in Gs, invert it, take the square root of it, and multiply that by what the trip time would be if you traveled at 1G. You then multiply that trip time by the acceleration in Gs to get the fuel usage (compared to 1G). So if something takes 3 days at 1G (3 Burn-days), and instead you go at 1/4 G, the inverse is 4, the square root is 2, so it will take 6 days. 6 days multiplied by 1/4 G is 1.5 Burn-days of fuel.

Similarly, you can accelerate to get to the destination faster, at a higher cost in fuel. Accelerating at 2Gs, the inverse is .5, the square root of that is ~.707, so that 3-day trip at 1G is now only ~2.12 days. However, you are burning twice as much fuel, so you use up ~4.24 Burn-days of fuel.

The fun question becomes how many days of life support you have on board. Each extra person-day assuming Bay Quarters masses 50 kilos*. Are you saving enough fuel to make up for the increased life support needs? Or would putting additional fuel into the cargo space be a better idea?

* Bays get 20 person-days of life support per ton of cargo allocated to life-support. 1 ton = 1000 kg. 1000 kg / 20 person-days = 50 kg per person-day.

Quarters get a better deal (5 kg per person-day), but require the up-front mass. If you are traveling for 106 days or less, see if you can use Bays and the 50 kg/person-day life support. If 107 days or more, Steerage Quarters are more efficient.

You can also perform the reverse calculation. If you have a distance that requires 5 Burn-days to cover, and your vessel can only carry 3 Burn-days of Fuel, you do the following:

Take the burn-days of fuel you have available (3)

Divide that by the number of burn-days needed (3/5 = .6)

Square it (.6^2 = .36)

That is the number of Gs you will be accelerating under (.36 Gs)

Working backwards:

Gs accel: .36

Inversion: 2.7778

Square root: 1.6667

Original trip time @ 1G: 5 days

Adjusted trip time: 8.3333 days

Trip time * accel = Burn-days used = 3 Burn-days

(I figured that by using math for Burn-days, the absolute value of the exponents involved would be smaller)

Hope this helps