Another long term (50 years) T1 here who's had on again off again background retinopathy for decades. First 15 years pre glucometer (and abysmal control in my teens). In my late twenties I was told that I'd need laser treatment by 40 but I'm still laser free at 59. (Have I been lucky?, probably/possibly yes, but I think those folk who start to get early complications after reasonable control are unlucky, outliers in the other direction). But ophthalmologists are very very used to treating diabetics now and blindness is a pretty rare end result - the eye tests are there just because they've learnt how to treat the eyes. Remember there are folk out there who maintain hba1cs above 10 for years - these are the ones who are risking serious diabetic complications.
My ratios certainly vary throughout the day, and I've got some insulin resistance. If I can get through the night OK (no hypos, no hypers) on my basal amount I'm happy, I know I need more insulin when I wake even though I just have coffee with a bit of milk for breakfast. It's easy enough to sneak a bit of extra insulin if my basal is lacking at points during the day. And exercise makes a massive difference to my insulin resistance, if I exercise just after taking bolus I know it will act more fiercely.
@Dave P would probably benefit from a cgm, if he's in the UK and testing 8 times a day or more he'd probably qualify for a free one. Also, have you considered moving to a pump? That would allow you to change basal rates during the day and allow for easier fine tuning.... (Though I suspect with an hba1c of 5.9 the clinicians will be reluctant to change anything, that 5.9 puts you firmly in the highest percentile of well controlled T1s). But one note of caution about frequent hypos for long term T1s. They can result in loss or reduction in hypo awareness, which is really bad news (and the reason why some clinicians get upset if they think your hba1c is "too low").
I've been officially type 1 for about 18 months, was type 2 before that, initially with good control on a low carb diet, which became worse and then bad control where I had to tolerate a few weeks of 24/7 double figure blood sugar before I was put on insulin.
You were probably never T2, but always a misdiagnosed T1/LADA (late onset T1). Because insulin production goes down gradually for late onset T1s they very frequently get an initial T2 diagnosis, and reducing carbs keeps their levels normal because their reduced insulin production can cope with a lower carb load. Eventually their insulin production goes too low and they suddenly get rediagnosed as T1, which in fact they were all along.
And as far as
reverse engineering those calculations go, you need to factor in two ratios
1) carb ratio (can vary by time of day and activity level)
2) correction ratio (which you haven't mentioned) .
Yesterday morning 7.2, ate 21g, 10u, at noon 3.3. Drop of 3.9.
This morning 5.5, ate 21g, 6u, at noon 8.9. Rise of 3.4.
final bg = initial bg - (insulin left over after amount needed for food)/correction ratio
Insulin difference between two days is 4 units, level difference is 7.3, which suggests you have a correction ratio of roughly 1 unit of insulin to 2mmol/L at that time of day (4:7.3). 9 units would probably have been enough yesterday, 8 units today. That does seem like a lot of insulin for just 21g but I suspect that that is the dawn phenomena for you. (Of course, I'm assuming that the DP is not affected by starting blood sugar and that your carb to insulin ratio is the same both days, which are not necessarily true. And there is the other factor that you may or may not be randomly producing your own insulin.)