what wavelength should I use to measure the transmittance of iodine colouring?
You know that in first approximation the absorption recorded is a product of the optical path length $d$, the extinction coefficient $\varepsilon$, and the analyte concentration $c$ as $$\mathrm{abs} = c \cdot \varepsilon \cdot d$$

Now, depending on your specific task, consider the three variables to your advantage.  Because for the wavelength eventually elected to record your data, you want to stay in a linear relationship between analyte concentration and detected absorption as the read-out.


The pathlength $d$ is something typically fixed to $1\,\pu{cm}$, but be aware that there are optical cells offering a shorter path length (e.g., $5\,\pu{mm}$, $1\,\pu{mm}$ as here or even $0.1\,\pu{mm}$) or longer paths (e.g., $50\,\pu{mm}$ (example), or $100\,\pu{mm}$.  It depends on the design of the spectrometer if mounts are interchangeable, or not, to benefit from an intended adjustment to your needs here.
What is the range of concentrations $c$ to record?  As the extinction coefficient depends on the wavelength of recording, it may be beneficial to record at a wavelength maximizing $\varepsilon$ of your amylose iodine complex, provided you want to detect low concentrations of this specis.  (Of couse it shouldn't be in the range other molecules of your solutions absorb intensely.)

But: if you want to determine solutions with a high concentration of your analyte, than it makes sense to record the absorption at a wavelength where $\varepsilon$ of your analyte just is high (not maximal), while «all the rest» of the sample solution does not contribute significantly to the absorption.  This approach will guard you to encounter absorptions too high to be sensible for detectors in the spectrometer.  Look up the manual of the spectrometer or ask colleagues already using the instrument; for some, an absorption greater than $2.0$ equates to «skip it, there is no light on the detector at all».


The iodine-amylose complex is known to absorb intensely.  This equally may justify to record $\mathrm{abs}$ in function of analyte concentration $c$ a little «off» from $\varepsilon_\mathrm{max}$.  By recording the spectra of your blank, and of a typical solution with your complex to quantify, you may justify how many nanometers above or below $\lambda$ yielding $\varepsilon_\mathrm{max}$ of your analyte is fine.  If time and resouces permit, you may record at multiple wavelengths, too; eventually, the readings should lead to the same concentrations per sample.