The key IMHO to a proper fit is to buy your boots / shoes in a shop with a salesperson who KNOWS their product. This person can better guide you if they understand the characteristics of each of the shoes you consider. Also, they can help you avoid making a mistake you may later regret. For this reason, I advise against internet shopping, at least until you are completely certain of your sizing for the made and style footwear you seek to purchase.
In particular, when speaking to the salesperson ask, which brands and models / styles run slightly larger or smaller than the stated size?
For example, Keen boots and shoes DO VARY from style to style. For example, my Keen Targhee II mid-high boots run one-half size SMALL. This according to the manufacturer's web site. Also, my Keen Arroyo II hiking sandals run one-half size SMALL, again according to the manufacturer's web site description.
The point being, exercise due diligence and do advance research.
Also:
1. Try to buy your footwear after you have been on your feet all day. They MAY have swollen slightly. This can approximate the effects of daily swelling while on Camino.
2. Try on the new footwear wearing the SAME SOCK combination you plan to wear while on Camino.
3. Make sure your heel is well seated in the heel counter and does not slide forward. Adjust the lacing accordingly. However, it is NORMAL for the heel to lift up slightly as you walk. However, the up and down movement should not exceed about 1/2 inch or 2 cm.
4. Finally, you should be able to wiggle and / or cross your toes in the toe box of the shoes and boots. Having this much space allows for swelling and, when properly laced-up, prevents your toes from slamming into the front of the toe box on a downhill segment. Remember, for every UP on the Camino, there is a commensurate "DOWN." Some are more steep and treacherous than others.
This toe-bumping condition is regularly seen among pilgrims wearing too small hiking footwear. It results in bruised, and bleeding toes, and lost toenails...yuck!
I hope this helps.