Hi,
l can confirm that you do indeed need to set the filter switch to bulb to expose Tri-x properly.
This setting will rate it at 160ASA.
lve been shooting Tri-x with Nizo 481/561/801/481m/561m/801m for the past 20
years.
This is how you expose Tri-x correctly.
For Ektachrome 100D you set the filter switch to sun the colour conversion filter is removed automatically and the film is rated as 100ASA
Jamie