But the best styles, IMO, are Sercial and Verdelho. Sercial is dry and Verdelho medium-dry[1], IMO have more fascinating flavors, and tend to be more acidic than the sweeter styles, which as a soda drinker I especially like. Both are really good for extended sessions as they're not nearly as cloying as Port. And the Sercial in particular suitable for drinking by the glassful with dinner if you care to splurge.
And one of the greatest things about Madeira is that it's already completely oxidized. Opinions very, but as for myself I still enjoy bottles of Madeira that I opened years ago. By contrast, IME an opened bottle of Tawny Port starts to lose its punch after a week or so (on the outside). This is why you can still buy Madeira from the 1800s and even 1700s and still enjoy it today. The oldest enjoyable Ports (Ruby or Tawny) tend to only go back 50 or 100 years if they've been preserved well.
If you're not a regular drinker (like me) or are the sole drinker in a household (like me), and don't enjoy hard liquor (like me), then the long shelf-life of opened bottles is really nice.
The Historic Series Madeiras from the Rare Wine Co are good exemplars of the various styles
https://www.rarewineco.com/rare-wine-co-historic-series-madeira
I don't know if they still do, but they used to offer a 4 half bottle sample gift set.[1] Though both still sweeter and thicker (mouth feel) than a Sherry, with less of the distilled spirits-like flavor/smell coming through--which is why I dislike Sherry.
Traveling around Spain and Portugal you can find Vintage Ports that cost about $25 and will taste absolutely amazing (though I try to stick to above 20-year oaked tawny blends). Quite a few are not very sweet, and not very heavy. Also, Vintage Port has a considerable shelf life. The oldest currently available vintage is 1815.
Note also,
"As a supplemental unit of apothecary measure, the wineglass (also known as wineglassful, pl. wineglassesful, or cyathus vinarius in pharmaceutical Latin) was defined as 1⁄8 of a pint, or 2 fluid ounces (2 1⁄2 fluid ounces in the imperial system).[11][12] An older version (before c. 1800) was 1 1⁄2 fluid ounces.[13] These units bear little relation to the capacity of most contemporary wineglasses, or to the ancient Roman cyathus."
The earlier version is 45mL.
(To nearest 5mL)
A bottle is 750 ml, so that's a 150-ml pour (five to the bottle) or 187.5-ml (four to the bottle). As the article says a typical glass holds about 450 ml now.