Flickering light during meditation

Nimitta

In the grand scheme of things its not particularly important, but it can serve as a sign to tell you that you have attained to a certain level of concentration. Past a certain point, the light doesn't flicker, its persistent and extremely blinding. What to do when nimitta arises really depends on your practice and what you are trying to accomplish. Some would tell you to focus your attention on the nimitta to reach deeper states, others will tell you to ignore nimitta and don't get distracted by it. At this point, if you're serious about pursuing meditation beyond a cursory level, you would greatly benefit from the guidance of a trained teacher.

Your guess answers your question. When we mediatate you already know that there are two sides. One side you will be discouraged and make you you to doubt (Sakhaya) about the method you are follwing. The other side will motivate you to stick to your method and not to open your eyes till your goal.

As @cgtk mentions, everything you feel ,see is a hint, sign that you have attained a certain level. But this is not always true as the subconscious mind or the Wizard likes to hallucinate, create illusions. If someone tends to follow/focus this flickering or any other sensations, it is to believe that you would be totally distracted from your ultimate goal and go for secondary benefits.

The best practice is to ignore and if it repeatedly coming, you can Maitri it and follow your meditation. If you had experienced it on the first day and if it feels great, do not expect it to happen on the second day as you are expecting means that it will most likely a illusion from the wizard (Vijñāna).

When you are following a meditation it is always the best to get (Kamatahan/kammaṭṭhāna) guidance from someone who is superior to your level. This helps you to be in the correct path and you can always share what you experienced as the superior teacher precisely knows what you are telling and the teacher will guide you for your ultimate goal.

Of that which is experienced during meditation, the Buddha says:

MN64:10-12.1: Furthermore, as the placing of the mind and keeping it connected are stilled, a mendicant enters and remains in the second absorption …
MN64:10-12.2: third absorption …
MN64:10-12.3: fourth absorption.
MN64:10-12.4: They contemplate the phenomena there as impermanent …
MN64:10-12.5: They turn their mind away from those things …
MN64:10-12.6: If they don’t attain the ending of defilements, they’re reborn spontaneously … and are not liable to return from that world.

Grasping at impermanence only leads to more grasping. Simply observe and relinquish without grasping.

It's just an apparition of the mind and nothing particularly important. Things like this will arise from time to time. Like Max said in the comment, the mind subconsciously tries to entertain itself. This actually happens, like clockwork, at set times in a sit the first usually occurring right around the the 20-25 minute mark. This is where we really begin to shift from ordinary discursive consciousness to something resembling a meditative state. Other phenomenon you might experience are feelings of your body twisting, your head or hands inflating, your muscles shaking involuntarily, your face disappearing, or, like you mention, your body feeling like it's made of stone. FWIW, I would actually say that the feeling of being made of stone is more indicative of progress than the light. If you can sit unmoving for even longer, a similar shift will happen at around the hour and fifteen minute mark when the body/mind's natural energy begins to arise. This is where things get interesting.

You're right at the cusp of passing beyond the beginner stage. If you can find a way to commit yourself a little more, you're practice will start to have a real impact on how you live your life and experience the world. My advice to you is to sit as stone for as long as you can. Try to push your sits towards the hour mark and see what happens.