First of all the sight picture shown at :
is not the "ideal" sight picture but merely an illustration of its elements.
Second point is to bear in mind that the primary requirement of the sight picture is to obtain good focus on the foresight. You should not compromise this to obtain a clearer image of the aiming mark. If you cannot get a usable aiming mark image then you probably require a corrective lens. If you are already using one then you probably require the correct one. This is not the same as the prescription for normal vision correction and if necessary you should seek the advice of an optician knowledgeable in the requirements of shooting.
It is vitally important that your head position is consistent and this is where having the correct eye relief is critical. The figure of the foresight tunnel occupying 1/3 of rear aperture is a guide, but no more than that. However if your sight picture is markedly different then you need to have a good reason for it. I disagree with the range of eye relief quoted above, you should be looking for a figure of the order 40-60mm. If a shooter had an eye relief even approaching the 150mm suggested then a coach really ought to be looking at other aspects of the rifle set up.
The rearsight iris is an interesting; and misunderstood; little piece of equipment. Altering adjusts the amount of light reaching the eye. this is critical because you require the best possible contrast between the black of the foresight and the white ring surrounding the aiming mark. too little or too much light will reduce the contrast and result in lower aiming accuracy. However the rearsight aperture also has as secondary attribute in that it acts in a similar manner to a camera lens aperture and has an effect of the apparent depth of field. This can help improve the aiming mark image, but bear in mind that it is the foresight that is paramount. The range of rearsight sizes is normally quite small, in the region of 0.9-1.2mm. Again, if you find your self straying far outside those figures then you should be asking why it is necessary.
Coloured filters are another area of some controversy. Whilst you should not need them indoors, some people do find them useful. bear in mind that all lenses reduce transmitted light; even clear ones. Certain eye conditions benefit from the use of a filter with a yellow tint as this does improve contrast. A filter can also help with the bright light associated with the increasingly common electronic targets; e.g. Meyton and Megalink. Filters can be worth experimenting with, but make sure that you evaluate their effect properly.