This is not straightforward, there are differences between the two calculators, however I have split the difference to get 30% viability for your 2 smackpacks and 300Bn cells required, you have a couple of options using both your smackpacks:

Stir Plate (I like Kai Troister so I'm using his calculations), a single 1.75 litre starter will give you 304Bn cells, that should be enough for your batch.

Intermittent shaking - 2 step starter 1st is 1.5 litre, 2nd 1.5 litre will give you 304Bn cells.

No shaking - 2 x 2 litre steps will get you 264Bn cells so maybe a little shaking when you can will help things get up to 300Bn or 2 x 1.5L steps to get you 216Bn cells and buy another smackpack, there is no guarantee that a new smackpack will be fresh enough to get you 85Bn cells, it would have to be dated 23rd June or later.

So the answer is to use/buy a stirplate not buy more yeast

Second best is to step them up with as much shaking as possible, third option is to shake as much as possible and buy another smack pack to be sure.