Even quant gurus can trip up when doing mental math while giving a presentation. As a result, it’s a good idea to avoid setting yourself up to do complicated math during your initial meetings with VCs.

One of the best ways to avoid this situation is to have backup slides that detail the underlying math of key analysis, such as COGS or the addressable market size. I find that entrepreneurs get into trouble when they have a slide the presents a summarized version of the calculation. These summaries beg questions about the underlying inputs, leaving you to start doing mental math while in the hot seat.

To further avoid doing mental math, you may want to respond to questions about inputs (that you’re not extremely comfortable with offhand) with an offer to send the document that illustrates the calculation after the meeting. While in the rare case where that offer doesn’t quell the investor’s request, you should be prepared to open the supporting documentation and explain the math. The need to open navigate through supporting documents during a presentation of a high-level overview of your business, however, is unlikely.

In sum, you should be prepared to be asked about the inputs to key calculations that are part of your business case. You’re likely to be best suited to answer these questions by providing detail in a slide or by sending follow-up information. Either way, try to avoid doing mental math when you’re in the hot seat – your nerves may get the better of you.