FeeOnly's advice is good, but I believe there can be an inherent bias in the 'ratio'
In Calgary, City performed property assessments are actually BASED on sale prices of comparable units (comparable in terms of location, size, upgrades etc). So actual sale prices and assessment prices are linked.
A ratio we used when putting an offer in was actually the (sale price)/(listing price) of comparable units (location build quality etc). This gives you an idea of what the market has been doing over a set amount of time.
How we got this info was asked our realtor to pull all the sales in the community incl. MLS listing histories (i.e. have the listing prices changed up or down over the past few weeks/months) and the final sale price. Since we are in a duplex, and they are relatively rare in our community, I also asked our realtor to pull duplexes in neighboring communities for the same info.
This gave us an idea of how much variation between asking and final sale price - in our community at the time, sale prices were about 95-98% listing price. So our initial offer was about 90% and we settled at about 93-94%.
This is almost exactly the same as what FeeOnly describes, however you need to know if the assesment price is accurate. In our case it wasn't. The assessed value was nearly 20% higher than what the market value was.