Learn Farsi: میزان موفقیت در ساختن این آلیاژ پنج در هزار است. - The success rate in making this alloy is five in a thousand.

Yes! Just as درصد (dar sad) literally means 'in a hundred' (percent), پنج در هزار (panj dar hezâr) literally means 'five in a thousand'. Farsi uses the preposition در (in) to express all statistical proportions this way, allowing you to easily swap numbers to say things like 'one in a million' (یک در میلیون).

Could I use درصد (percent) instead of میزان (rate) at the beginning of the sentence?

It would sound a bit contradictory here. درصد specifically means 'percent' (per hundred). Because the statistic in this sentence is out of a thousand, میزان (mizân), which broadly means 'rate', 'level', or 'amount', is the correct term to use for 'success rate'.

Where do the spoken Ezafe syllables (the 'e' sounds linking words) go in this sentence?

You will hear two of them: میزانِ موفقیت (mizân-e movaffaghiyyat) and ساختنِ این آلیاژ (sâkhtan-e in âliyâzh). Notice there is no Ezafe after موفقیت because it is followed by the preposition در (in), rather than another noun or adjective.

Why is the full infinitive ساختن (to make) used after در instead of a conjugated verb?

In English we say 'in making', using an '-ing' word. In Farsi, when you need a verb to act as a noun right after a preposition like در (in) or بدون (without), you simply use the full infinitive, which is the dictionary form ending in .