Hi again Janine,
To adjust your recipe oil weight in Soapcalc, you can click option 2 "Weight of Oils" and click on the "grams" button. In the grams field you can enter 250 or 300 grams, and then check the "view or print recipe" to see if the total batch weight is what you are after, then adjust from there.
To check your mold volume, fill it up with water, to the amount you want to make, and then pour the water into a measuring jug. (*Correction) The volume will be the same, but the weight of your soap batter will be different to the weight of your water.
In steps ...
1/ Fill you mold with rice or water to the level you want to measure, then pour this into a measuring jug to find out the volume in millilitres
(it will be roughly 1.5 litres to the top, so measure to the level you actually want)
2/ Multiply this amount by 0.70, this will give you the (rough, but close) weight of the soap batter you need to make
(eg. to the top will be roughly 1.5 litres x 0.70 => approximately 1 kilogram)
So, if you make a batch of soap that weighs about a kilogram, it should fill your mold (you will be able to refine this once you've made the recipe once ... there will always be adjustments).
Personally I would make a 400gram batch of soap (total weight of soap before cure), and put the dam wall so that you are filling half of your mold.
(*edited to correct error)