Your first decision to make is what you are going to shooting at, one size does not fit all. The first thing to choose is a magnification. In FITA field this magnification is going to be driven by your cheat system (rule of thumb 0.55 for 29” AMO draw length, 0.8 for 25” draw length with a fairly linear correlation between magnification and draw length). If you are shooting target you need to pick a magnification that will allow you to “frame” the target. This means that you almost definitely do not want a magnification so low that you can see a lot of the none-scoring part of the target face, heaven forbid the boss, through your scope. To choose a magnification try various lenses without any dot or aiming guide and see which gives you the best execution and grouping results. A side note for both field and target shooters is that you don’t want to be using extremes of sight bar extension as this causes inconsistencies left and right, where possible a sight extension bar between minimum four and maximum six inches is ideal.
So once you have picked your lens size (and don’t be afraid to experiment with up to 1.2 dioptre lenses to get the best frame) you need to pick an “aiming guide”. There are two major routes to go down with respect to a dot (and 90% of the top archers use a dot) but we will first look at other options: An open ring makes little sense on a lens which is framing the target. It tends to distract from the overall “picture” but if you have poor eye sight may well be the best option. For the same reason a combination of dot and circle is unlikely to be effective for aiming and execution and if it is your cheat system for field I personally would look at a different system.
Dot size, in an ideal world you want to be able to shoot the least distracting dot that you can and there is a simple test for this. Put the smallest dot that you have access to on your scope and shoot at 18m at a target pin (3 target pins if you don’t want to break arrows). You should be able to hold the dot on the target pin without a large amount of up and down movement. Because we’re using a target pin on a boss or a piece of paper there will be no opportunity to frame and so you can see how still the bow holds and your ability to focus on what you’re aiming at, not the dot. If this is good, transfer to the target face that you’re going to shoot and double check that you are still framing the target in the scope and you are done.
If you cant hold the dot on the target pin increase the size of the dot until you can, with the proviso that you are looking to hold the dot still, it doesn’t have to cover the pin that you’re aiming at but it must hold the same every time. Once you’ve selected a dot size transfer to the target face and for the first few arrows concentrate on what picture you will see through the scope with respect to the dot and the centre of the target and the overall frame. It is unlikely that if you need to go to a bigger dot you will be comfortable holding the dot in the dead centre of the target so it is all about the picture that you see and reproducing the picture you expect when you look through the scope.