**Assuming a typical UK domestic gutter: aim for about 30mm or less where practical, and treat 50mm as a maximum only if your gutter manufacturer permits it.** Neither figure guarantees that the gutter will catch the water: its width, position beneath the tiles and the roof pitch also matter.

FloPlast’s installation guide specifically says to fit the outlet **no more than 50mm below the roof tiles**. That is a maximum at the low point, rather than a target gap along the whole run. Without seeing your 30mm guide, I cannot tell whether it is a stricter recommendation or relates to a different system. [FloPlast installation guide](https://www.floplast.co.uk/uploads/related-documents/vqChg2jTaorA9euuY0SruczmrL0E1yXqi5peXcAm.pdf)

The main reason for limiting the gap is **water overshooting the front of the gutter**. Water leaving the tiles in heavy rain has forward momentum; lowering the gutter gives it more distance to travel outward before reaching gutter height. Marley explicitly warns against lowering the gutter so far that this happens. [Marley installation guidance](https://www.drainageonline.co.uk/Content/files/11025/Marley%20Installation%20Guide.pdf)

For your installation:

- **Use a modest fall.** FloPlast specifies a maximum gradient of **1:350**, approximately **3mm per metre**. Over an 8m run that means about 23mm of drop. A 20mm gap at the high end would therefore become approximately 43mm at the outlet if the tiles were level. [FloPlast FAQs](https://www.floplast.co.uk/company/frequently-asked-questions/rainwater-faqs)
- **Set a string line and check the gap along the entire roof edge**, including uneven tiles. Keep a consistent fall toward the outlet; following each tile’s irregularity could introduce dips.
- **If the required fall makes the gutter too low**, consider a central outlet or another downpipe to shorten the falling runs.
- **Check the horizontal overlap too.** The tile drip edge must sit comfortably over the gutter opening. A small vertical gap cannot compensate for poor horizontal positioning.

I would also fit the felt support trays while replacing the gutters if practical. Their purpose is to support the underlay and discharge water from beneath the tiles into the gutter; **they do not cure overshooting runoff from the tile surface**. Make sure the eventual tray edge will drain inside the gutter. [BMI underlay support tray](https://www.bmigroup.com/uk/p/underlay-support-tray-2479515345/)