Getting a fixed R-square value from linear trendline

Maybe thisin C2, copied down:

=CORREL(A2:$A$2325,B2:$B$2325)^2

and scroll down till you find 0.99

You may need ; instead of ,

	a	b	c	d	e
248	99.867	0.770
249	100.267	0.772		330.0667	d249: {=index(a250:a2325, match(2, 1/((c250:c2325 >= 0.99) * (c251:c2326 < 0.99))))}
250	100.867	0.774	1.00000		c250: =rsq(a$249:a250, b$249:b250)
251	101.600	0.775	0.97050
252	102.267	0.777	0.98823
253	103.200	0.777	0.90185
254	103.800	0.777	0.84443
255	104.467	0.778	0.87779
256	105.000	0.778	0.90321
257	105.400	0.779	0.92645
258	105.867	0.781	0.93836
259	106.267	0.783	0.91688
260	106.667	0.785	0.88855
261	106.933	0.786	0.88177
262	107.133	0.790	0.84443
263	107.467	0.791	0.83395
264	107.867	0.795	0.82108
265	108.333	0.795	0.83409
266	108.733	0.799	0.84020
267	109.133	0.801	0.85096
268	109.667	0.803	0.86385
269	110.067	0.805	0.87647
270	110.667	0.807	0.88872
271	111.133	0.809	0.90097
272	111.733	0.810	0.91272
273	112.333	0.812	0.92305
274	113.133	0.813	0.93198
275	113.667	0.814	0.93852
276	114.267	0.815	0.94353
277	114.733	0.816	0.94766
278	115.133	0.816	0.95067
279	115.467	0.819	0.95482
280	115.800	0.820	0.95841
281	116.200	0.823	0.96195
282	116.600	0.824	0.96502
283	117.067	0.827	0.96780
284	117.533	0.828	0.97028
285	118.067	0.831	0.97252
286	118.533	0.832	0.97459
287	118.933	0.834	0.97643
288	119.333	0.836	0.97798
289	119.667	0.838	0.97938
290	120.133	0.840	0.98061
291	120.667	0.842	0.98182
292	121.200	0.844	0.98291
293	121.600	0.846	0.98382
294	121.800	0.848	0.98433
295	121.933	0.854	0.98311
296	122.200	0.855	0.98232
297	122.667	0.858	0.98148
298	123.333	0.856	0.98216
299	124.067	0.859	0.98299
300	124.733	0.861	0.98385
301	125.533	0.863	0.98476
302	126.267	0.863	0.98559
303	127.000	0.866	0.98639
304	127.600	0.868	0.98710
305	128.067	0.870	0.98780
306	128.333	0.874	0.98843
307	128.533	0.879	0.98861
308	128.867	0.879	0.98893
309	129.200	0.883	0.98887
310	129.667	0.883	0.98916
311	130.133	0.886	0.98936
312	130.800	0.884	0.98986
313	131.600	0.887	0.99035
314	132.267	0.884	0.99046
315	132.933	0.884	0.99024
316	133.400	0.886	0.99001
317	133.800	0.886	0.98970
318	133.933	0.887	0.98958
319	134.067	0.893	0.98997
320	134.333	0.899	0.99030
321	134.733	0.899	0.99066
746	329.400	1.807	0.99013
747	329.600	1.815	0.99009
748	329.800	1.817	0.99006
749	330.067	1.825	0.99001
750	330.467	1.829	0.98996
751	330.867	1.835	0.98990

??

Put the formula that's shown for D249 in D249, and the formula that's shown for C250 in C250 and copy down.