"Zero point first" is definitely wrong. I think you should say "the zero-point-one-th percentile", just as you say "x to the n-plus-one-th". Your spelling checker will mark this as wrong, but it is the only reasonable possibility.

TL;DR

I suggest reading it as a mixed fraction. Here's the formula:

(Mixed-number ordinal) = (integral-part ordinal) + ("-and-") + (Hyphenated, fractional part)

Examples

Numeric Percentile | In Words (Fractional denominators are powers of ten.) +--------------------+-------------------------------------------------------+ | 0.1 | zeroth-and-one-tenth percentile | +--------------------+-------------------------------------------------------+ | 23 | twenty-third percentile | +--------------------+-------------------------------------------------------+ | 23.05 | twenty-third-and-five-hundredths percentile | +--------------------+-------------------------------------------------------+ | 10.001 | tenth-and-one-thousandth percentile | +--------------------+-------------------------------------------------------+

Explanation

Consider the number 1.5.

As opposed to reading the decimal number digit-by-digit (i.e. one-point-five), one can also read it as a mixed fraction (i.e. one-and-a-half or one-and-five-tenths).

When we use the conjunction and to join the integral and fractional parts, we have essentially created a compound noun; one where the integral part is the main noun, and the fractional part describes the integral part. Hence, only the integral part needs to change to ordinal form.

I suppose one could also say zeroth percentile and a half, but that stylistically feels awkward to me.

I understand mathematically speaking this is impossible, but as to reporting on measures, my supervisor uses "below the 1st percentile" in written explanations. Prior to narratives we supply precise numbers in a table.