In terms of your question. The hardcoded value in cell I54 (i.e. Equity amount) is intentional. That value is updated via the ‘Click to Recalculate’ button, which when clicked runs a Goal Seek VBA macro that iteratively changes the value in cell I54 until the target LTC (cell C55) equals the actual LTC.
You can test this by a) opening a fresh copy of the model, b) changing the LTC input in cell C55, and then c) clicking the red ‘Click to Recalculate’ button. You’ll see once the recalculate operation runs, that the value in cell I54 has changed.
Spencer uses this logic/technique to avoid the circular reference issue inherent in construction interest calculation. So there is no need to make any changes to the value in cell I54 – in fact, adding a formula in cell I54 will break the model.