From Eldredge, Debra M., et al., 2007 Dog Owner's Home Veterinary Handbook, 4th edition. Howell Book House, Hoboken, New Jersey.
Dog years: Human Years (for a 21-50 lb. dog)
I didn't reproduce the entire table (it's Appendix B), but as you can see, it's not a straightforward 1 dog year = 7 human years. For smaller dogs (think Mr. D!), it's a smaller conversion factor (e.g., a 5-year-old dog less than 20 lbs is about 36 human years); for a big dog, it's a larger factor (over 90 lbs., a 5 year old dog is the equivalent of 42 human years).
So, trying to accommodate the conversion factor slowing as the dogs age made me hazard that at 40, Belle would be about 180 human years. Give or take a decade (this is where I wink at you).
This means Stinkerbelle is about 56 human years right now, and Mr. Big, who has lost 8-10 lbs. since he started his diet last fall, is 66 in human years.
And I am, in canine years, 6 (if I was a 51-90 lb dog). Hooray!!!