Приветствую, Илья! Да, конструкции типа a third more / less грамматически правильны. Вот несколько примеров из СМИ:

In our model, forests outside the protected areas lost up to a third more species from climate change and deforestation.

Last year, investors purchased $124 billion, nearly a third more than the previous peak in 2006.

Luxury handbags, which might sell for $1,000 to $5,000, cost on average nearly a third less in Europe than in mainland China

Напомню, однако (и это есть в учебнике), что «наполовину больше» переводится иначе, а именно — half again as much, например:

A notebook still typically costs half again as much as a similarly equipped desktop.