Buying Limestone to Produce Cement Faster? – Two day Comparison

I needed to level up before I could build my next factory and that would take two days.
So I had two days to get 150 cement. My factories are at AE4 and my entrepreneurship is 45.
The values below is based on that.
This can be applied to Iron and Steel as well. (if we stick to building materials)
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When preparing for a new factory, there are two ways to build up the required cement stock:

Option 1: Produce your own limestone and split your work actions between limestone and cement.

Option 2: Buy limestone and use all of your work actions directly for cement.

Let's compare them over two days, using a target of 150 cement.

Assumptions

• Automatic cement production: 10 cement/day

• Own produced limestone → manual production: 20 cement/day

• Bought limestone → manual production: 40 cement/day

• 10 limestone = 1 cement

• Limestone price: 0.08 BTC

• Cement price: 1.586 BTC

Option 1 – Produce your own limestone

Over two days:

• Automatic: 10 × 2 = 20 cement

• Manual: 20 × 2 = 40 cement

Total: 60 cement

You still need:

150 − 60 = 90 cement

Buying 90 cement costs:

90 × 1.586 = 142.74 BTC

Option 2 – Buy limestone

Over two days:

• Automatic: 20 cement

• Manual: 40 × 2 = 80 cement

Total: 100 cement

You now only need:

150 − 100 = 50 cement

Buying 50 cement costs:

50 × 1.586 = 79.30 BTC

The additional 40 cement requires:

40 × 10 = 400 limestone

400 limestone costs:

400 × 0.08 = 32 BTC

So total spending is:

79.30 + 32 = 111.30 BTC

So under these assumptions, buying limestone saves 31.44 BTC while producing 40 additional cement over the two days.

This doesn't necessarily mean it is always the better choice. Market prices, available cash and what else you could use your money for can all change the calculation.

Am I missing something? Any logical fallacy?