If I do the math right, it should be 1:60 to 1:70
Guy of approx. 170 cm (standard european) goes for 1700 mm. If he´s down to 28 mm he is scaled pretty exactly the sixtieth part of his original size.
If you do that with 1800 mm Original downsized to 28 you should reach 1:64, so this would be a good guess.
If you go for a Person with another 10cm more for a helmet reaching 1900 mm now you reach 1: 68 and with a big guy and helmet reaching 2000 mm it is approx 1: 71,5
So my guess reach into the middle !
1:65 (that reaches the height of an average male from the netherlands with a height of 1,82 m)
Just looked up the standards of the modelling range, take the common 1:72 models and you won´t be too far away (see above).