Gauging by the number of sorties flown, and thus about the number of aircraft available to the Assad-Regime on average: yes, that’s quite accurate.
Before the US strike on Shayrat AB, the SyAAF had some 3–4 operational MiG-21s at Hama; about 10 operational MiG-23ML/MLDs and MiG-23BNs at Hama, Shayrat and Dmeyr; MiG-25s were out of service; MiG-29s hardly ever flown; but there were up to 16–18 Su-22s at Shayrat, Tiyas and Dmeyr. Up to 20 L-39s — of which about a dozen was operational — rounded up the picture; with remark that these are slow and can carry only a friction of weapons load carried by other types.
These 50–60 aircraft were flying between 45 and 60 sorties a day before 7 April.
Ever since, they’re flying 20 sorties (max 30, like yesterday, when L-39s are flying a lot).
Furthermore, clear evidence (photos and videos) is meanwhile available for destruction of at least 9 MiG-23MLDs and Su-22s at Shayrat.
Thus, I would go as far as to say: yes, 20% of available aircraft were destroyed. Even more so: destruction of these 20% aircraft resulted in 40% decrease in number of sorties flown.