A global average is not 10%. I've seen conversion rates of 1% in some sectors. There is no such global average.
If you are truly struggling then yes ask the person how many people convert. He or she will probably say that x amount of visitors come in and y amount actually transact. Do the mental arithmetic and calculate the conversion rate right there.
If this is not possible, ask to see the Analytics. For sectors that do not have tangible products for sale, the best way to do this (if they have an existing website) is to base the conversion rate by dividing the total amount of unique visits by the number of people that visited the contact page. To further improve this, or tighten up the value, you could check how many people clicked the submit button on the contact page versus the amount of unique traffic to that site over a 3, 6 or 12 month period. If this does not exist, then offer to setup conversion tracking prior to starting your campaign with them. Run it for 1 month, and then tell them how much you can improve their conversion in terms of real money. Always be sure to tell them the estimated value of the increase in conversion.
Failing this, as I said previously, in the original post where I gave everyone the calculation, use some logic. A business selling a service that costs in excess of a $5000 will generally not have a higher conversion rate than 5%. At least it's very rare.
But here are some conversion rates that I've personally witnessed in certain areas.
3rd Largest Office Supplies/Printer Consumables e-store: 12% Conversion rate (SEO/PPC/SMO/Display Advertising/CSE's)
Private Dentists: 5% (SEO/PPC/Local SEO/Offline Marketing)
Public/Private Dentists: 8% (SEO/PPC/Local SEO)
Cosmetic Dentists: 3% (SEO/PPC/Local SEO/Display Advertising)
Accountant: 11% (SEO/Local SEO)
Solicitors: 6% (SEO/Local SEO)
Online Retail (Electrical Goods): 9% (PPC only)
And finally, you should not be running through the excel sheet with the client. It's a slightly noobish thing to do, what you should be doing is showing them two things. Where they are right now, and where they will be if they carry out a campaign with you. So ideally this sheet should be adjusted to hold a pie chart or line graph to show the actual change in business resulting directly from your work with them. No one cares about estimates and global averages, people care about where they could (or will) be.