I just analyzed the sales in Wellesley in the past 3 months in the price range you specified ($600-$800). Of the 23 properties 6 had a lower sale price versus the assessed and 17 had higher sale prices.
Of the homes that sold below the assessed value the "sale to assessed" range was 75.3%-98.9% and for the ones that sold above the assessed value the range was 101.6%-129.3%.
I have a spreadsheet that I just put together and would be happy to share it with you and then you can analyze the data any way you wish: number of bedrooms, lot size, etc.
I would like to point out, however, that I would not focus on the assessed value. For one thing, this value is revenue based and not market based. Town assessors are not able to visit every single property and evaluate both the interior, exterior and lot amenities and this can result in both over and under assessments.
For instance, one property that sold significantly below the assessed was a home on Maugus Hill. Great neighborhood, great house and on the surface a great lot at 30,000 sf. However, what the assessment did not take into consideration is the lots elevation. Huge drop off in the back yard--while great views, not desirable to many buyers.
On the reverse side there was 4 Ingersoll Road. It is assessed at $738,000 but sold for $785,000 or a 106.37% sale to assessed value. Again, what the assessor was not able to take into consideration was the lot--it was one of the most level and private 15,000 sf lots in the area and was also located on a quiet dead-end side street. My partner and I both listed and sold this home. When determining the price we knew that the location, lot and the flow of the home would have strong appeal.
Hope this helps.......feel free to call me at 339.222.8103 with any questions.