As mentioned, finasteride inhibits virtually the same amount of DHT for all doses of around 0.2mg upwards. And if you were to keep reducing your dosage until you eventually find a dose that minimizes your side-effects, in all likelihood this dose will not be doing much for your hair loss.
The equation works like this: reduce your dosage = less DHT being inhibited = less side-effects = less hair gains. This is how it works, and there isn't any way around this unfortunately.
If you got bad sides from finasteride, which does occasionally happen, you're fighting an uphill battle unfortunately. No other treatments that we have available are anything like as effective as finasteride/dutasteride. Your best option is probably to give RU plus CB a try, and maybe also start taking a low dose of oral minoxidil.
The reason for this is that a very small number of people are outliers, who need a higher dose than usual in order to potently inhibit their DHT. In order to take account of these outliers, Merck decided upon a dose of 1mg for hair loss. For the vast majority of people however, there would be essentially no difference between a dose of 0.25mg and a dose of 1mg.